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PostPosted: Thu Sep 21, 2017 1:49 pm 
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walt wrote:
Retired guy (me) must be a little bored this morning ... but I played with some numbers given my peak speed with a 2.5 hp outboard and Husse0416's peak speed with a 5 hp outboard.

Listening to H's video, I think the 5 hp outboard was running up into the proper RPM range for peak HP and not rev limiting. So Im assuming that both he and I were getting in the range of the rated HP. He did not have the mast on (I did) and I dont know if the pedal drive was in place (mine was) and his boat is a little heavier from the add on.. but its a close enough comparison. In both cases, one person in the boat sitting in the back seat.

H didnt give the exact peak speed (a little over 9 knots) and for this little exercise, I assumed H's peak speed was 9.3 knots. My peak speed was close to 8 mph or 6.95 knots.

We know that hull drag follows a square law with speed at low speeds and then the exponent increases at higher speeds for a variety or reasons. Anyhow, we have two data points and in this case, assume an exponential equation for curve fitting the two data points. For sure a linear equation does not work for this case.

Details available if someone wants but the exponential equation that fits those two data points is

Speed (knots) = 6.956 * ((hp/2.5)**.4188 )

Note that a square law would have an exponent of .5 and a cube law would have an exponent of .333. So on the TI, the speed vs hp required for that speed is somewhere in between square and cube.

Using this equation, I plugged in some different HP numbers to see what speed would be predicted. Note that the numbers for 2.5 and 5 exactly match the real data.

HP / Knots
-------------
.625 / 3.89
1.25 / 5.203
2.5 / 6.95
5 / 9.3
10 / 12.43

That equation only is accurate near the two exact data points at 2.5 and 5 hp but its somewhat interesting to look outside the range. Defy here https://www.hobie.com/forums/viewtopic. ... 8&start=45 was getting 5.1 mph with the 400 watt Torqueedo at peak power (didnt post what power the control said was being used, that would certainly be interesting). Note that the equation based on the 2.5 and 5 hp data points predicted that 466 watts (.625 hp) would only get up to 4.47 mph or 3.89 knots and Defy was going faster than that. I take that to mean that at the lower speeds, the exponent goes back closer to square law of the typical parasitic drag vs speed equations.

Also.. its likely that the equation over estimates the speed for 10 hp as the exponent is probably lower for 10 vs 5 or 2.5. But I think its fair to say that you will not get speeds over 12.43 knots or 14.29 mph if for some crazy reason you did put 10 hp on a TI.

Ok.. back to doing something more useful (or maybe defending all that junk.. which could be incorrect).



LOL!! LOVE IT!
Thanks for clearing this up... I didnt understand half of it :roll: but it sure looks very convincing. :P
The speed on my TI with the 5Hp, mast and all, is about 9knots at about 60% trottle. Pedals in the rear cockpit. Speed only increases marginally to just under 11 knots on full throttle but it probably is doubling the fuel consumtion.
About half throttle is perfect crusing speed if the water is calm. To make it go faster I guess i could try to add some more hydrofoil fins on the hull but that is a new separate project far ahead in the future.
It will be interesting to see how the 5Hp handles the extra weight that I will soon be adding with all the batteries and the electric motor (~50kgs extra). All of that weight is in the front. The drag will for sure increase but I hope the kayak will get better balance and "plane" better.
(Well it actually never will fully "plane" off course but I hope you all get my meaning) :)

Range is more important for me then speed so I hope it handles well even when fully equiped and loaded.
So far I am very happy with the setup. :D

I am the only guy around with a Hobie Island here in South of Sweden... it sure would be nice to meet up with some Islanders but you are all to far away.

Thanks again for the scientific analysis!
:)

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Sailing my TI and fishing.... thats bliss!!


Last edited by Husse0416 on Fri Sep 22, 2017 1:11 am, edited 3 times in total.

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PostPosted: Thu Sep 21, 2017 1:52 pm 
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Defy wrote:
I will have to take some pics of the controller next time i am out. But i pretty consistently get about 5mph at full throttle.


Sent from my iPhone using Tapatalk


What range/runtime are you getting with your electric motor?

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PostPosted: Thu Sep 21, 2017 2:06 pm 
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mswlogo wrote:
I'm waiting for the chapter of Husse to start boring out the hull for more storage space ;)

I'd never do anything like this to my T.I. but it sure is entertaining to watch from my "dry" desk.


LOL!! Glad you can enjoy my build and mods. :D
Well.. as you are mentioning storage space....

"We`re gonna need a bigger boat!!"
Tada..tada... tadadada...
:mrgreen:

I am accually thinking about trying to tow a kayak after my TI for loading some gear and to use as a secondary kaya on longer trips.
I will test to tow my little china yak next season. I have heard that towing kayaks can be a bit problematic as they tend so zigzag a lot?
I have never tried it. Was thinking about rigging a rudder controlled by the towline if necessary to make the towed kayak follow better in line and nit zigzaging.

All the best!
/Gustav

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PostPosted: Thu Sep 21, 2017 2:28 pm 
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Posts: 380
Husse0416 wrote:
Defy wrote:
I will have to take some pics of the controller next time i am out. But i pretty consistently get about 5mph at full throttle.


Sent from my iPhone using Tapatalk


What range/runtime are you getting with your electric motor?


Full throttle with the stock battery (320 wh) i get about 3 miles/30ish mins. At 50% throttle i can get 6-10 ish. So my guess once i buy the 915wh battery i will get about 10 miles out of it at a nice speed.




Sent from my iPhone using Tapatalk


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PostPosted: Thu Sep 21, 2017 3:05 pm 
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Defy wrote:
Husse0416 wrote:
Defy wrote:
I will have to take some pics of the controller next time i am out. But i pretty consistently get about 5mph at full throttle.


Sent from my iPhone using Tapatalk


What range/runtime are you getting with your electric motor?


Full throttle with the stock battery (320 wh) i get about 3 miles/30ish mins. At 50% throttle i can get 6-10 ish. So my guess once i buy the 915wh battery i will get about 10 miles out of it at a nice speed.

Sent from my iPhone using Tapatalk


Sounds encouraging!
If I load both my battery packs I have about 2400WAh. I will mostly use my trolling motor for positioning and using the gps anchor lock. Charging is always hard when out camping so I hope I can make the batteries last for a week if I use them carefully.
:)

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PostPosted: Thu Sep 21, 2017 3:35 pm 
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Location: Colorado
Quote:
Speed only increases marginally to just under 11 knots


Woops, I used 9.3 knots running those numbers partly from watching the video where you said the top speed was about 9 knots.

Curious, did you change something from when the video was taken.. 11 knots is a HUGE amount over 9 knots..

A link to your video again where I got the 9 knot comment (which looks like you are really moving along..)

https://www.youtube.com/watch?v=lO3D9vCesAc


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PostPosted: Thu Sep 21, 2017 4:37 pm 
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Husse0416 wrote:
Defy wrote:
Husse0416 wrote:
Defy wrote:
I will have to take some pics of the controller next time i am out. But i pretty consistently get about 5mph at full throttle.


Sent from my iPhone using Tapatalk


What range/runtime are you getting with your electric motor?


Full throttle with the stock battery (320 wh) i get about 3 miles/30ish mins. At 50% throttle i can get 6-10 ish. So my guess once i buy the 915wh battery i will get about 10 miles out of it at a nice speed.

Sent from my iPhone using Tapatalk


Sounds encouraging!
If I load both my battery packs I have about 2400WAh. I will mostly use my trolling motor for positioning and using the gps anchor lock. Charging is always hard when out camping so I hope I can make the batteries last for a week if I use them carefully.
:)


Yeah not sure if the trolling motors efficiency is the same but with that much battery and just for gps lock i would give it a week of light use.


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PostPosted: Thu Sep 21, 2017 10:54 pm 
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walt wrote:
Quote:
Speed only increases marginally to just under 11 knots


Woops, I used 9.3 knots running those numbers partly from watching the video where you said the top speed was about 9 knots.

Curious, did you change something from when the video was taken.. 11 knots is a HUGE amount over 9 knots..

A link to your video again where I got the 9 knot comment (which looks like you are really moving along..)

https://www.youtube.com/watch?v=lO3D9vCesAc


I did not go full trottle on that clip, sorry I should have pointed that out. About 75% trottle on that clip. To get the speed up over 10 knots even with full trottle I also have to reposition my weight (fat a** :lol: ) a bit forward and you can feel the hull fighting against its hydrodynamic properties.
It does not feel ok to strain the hull and outboard (you can hear it working hard) to the maximum so I decided that 9 knots is plenty for my purpose. :)

I am working on new sealed MD plugs with non-return scupper valves. Without totally sealing the front MD well I have a beautiful large water-fountain in the kayak that would make the Bellagio in Vegas very jelous! :lol: The stock MD plug unfortunately does not seal the well good enough.
Its ok in the rear MD well with the drive in place, no surge of water there, but if the drive is removed almost the same thing happens.
The Mirage drive fins are also under tremendous stress from the water drag when the speed is aproaching 9-10 knots so it would be nice to have better working plugs both front and rear. I will post once I have a workning sollution to the problem.
The scupper/cart holes in the rear "cockpit" must also be plugged otherwise the same "fountain-effect" happens.
No problem at all with the scupper holes in the rear cargo cold, they drain fine and does not backflow at all even att full speed.

All the best!
/Gustav

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Sailing my TI and fishing.... thats bliss!!


Last edited by Husse0416 on Fri Sep 22, 2017 7:19 am, edited 1 time in total.

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PostPosted: Fri Sep 22, 2017 12:49 am 
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I tightened up all the bolts in the anchor trolley this morning and made a Quick test. The line slides nicely so I guess it will work ok. When I look at the setup in the light of day I see that the metall Harken pulleys have a very small area on the outside of the kayak. This gives a rather big risk of damaging the plastic if the load on the anchor line gets to big.
So I will remount them today and add a stainess protection plate both on the outside and the inside to be sure the hull will stand up the all abuse without damages.
:D
Allways something to fix... but good fun!!
:mrgreen:

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PostPosted: Fri Sep 22, 2017 7:08 am 
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Husse0416 wrote:
What range/runtime are you getting with your electric motor?


Defy wrote:
I will have to take some pics of the controller next time i am out. But i pretty consistently get about 5mph at full throttle.


So do I. Stock TI users can expect a consistent 5mph from the Torqeedo 403 at full throttle. That can be improved slightly with a few tweaks which I covered in my 403 review, namely making the prop shaft more hydrodynamic and lowering the prop enough to stop cavitation. I can get a little over 6mph in favorable conditions but it is not consistent.


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PostPosted: Fri Sep 22, 2017 7:13 am 
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Husse0416 wrote:
Defy wrote:
I will have to take some pics of the controller next time i am out. But i pretty consistently get about 5mph at full throttle.

What range/runtime are you getting with your electric motor?

For detailed range information with the Torqeedo 403 see this thread.


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PostPosted: Fri Sep 22, 2017 7:49 am 
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walt wrote:
Retired guy (me) must be a little bored this morning ... but I played with some numbers given my peak speed with a 2.5 hp outboard and Husse0416's peak speed with a 5 hp outboard.

Listening to H's video, I think the 5 hp outboard was running up into the proper RPM range for peak HP and not rev limiting. So Im assuming that both he and I were getting in the range of the rated HP. He did not have the mast on (I did) and I dont know if the pedal drive was in place (mine was) and his boat is a little heavier from the add on.. but its a close enough comparison. In both cases, one person in the boat sitting in the back seat.

H didnt give the exact peak speed (a little over 9 knots) and for this little exercise, I assumed H's peak speed was 9.3 knots. My peak speed was close to 8 mph or 6.95 knots.

We know that hull drag follows a square law with speed at low speeds and then the exponent increases at higher speeds for a variety or reasons. Anyhow, we have two data points and in this case, assume an exponential equation for curve fitting the two data points. For sure a linear equation does not work for this case.

Details available if someone wants but the exponential equation that fits those two data points is

Speed (knots) = 6.956 * ((hp/2.5)**.4188 )

Note that a square law would have an exponent of .5 and a cube law would have an exponent of .333. So on the TI, the speed vs hp required for that speed is somewhere in between square and cube.

Using this equation, I plugged in some different HP numbers to see what speed would be predicted. Note that the numbers for 2.5 and 5 exactly match the real data.

HP / Knots
-------------
.625 / 3.89
1.25 / 5.203
2.5 / 6.95
5 / 9.3
10 / 12.43

That equation only is accurate near the two exact data points at 2.5 and 5 hp but its somewhat interesting to look outside the range. Defy here https://www.hobie.com/forums/viewtopic. ... 8&start=45 was getting 5.1 mph with the 400 watt Torqueedo at peak power (didnt post what power the control said was being used, that would certainly be interesting). Note that the equation based on the 2.5 and 5 hp data points predicted that 466 watts (.625 hp) would only get up to 4.47 mph or 3.89 knots and Defy was going faster than that. I take that to mean that at the lower speeds, the exponent goes back closer to square law of the typical parasitic drag vs speed equations.

Also.. its likely that the equation over estimates the speed for 10 hp as the exponent is probably lower for 10 vs 5 or 2.5. But I think its fair to say that you will not get speeds over 12.43 knots or 14.29 mph if for some crazy reason you did put 10 hp on a TI.

Ok.. back to doing something more useful (or maybe defending all that junk.. which could be incorrect).

This is very useful information if it is accurate, and from my experience with TI motors it appears to be. It also clearly shows the declining justification of adding more weight and horsepower to achieve only modest gains in maximum speed. A slight gain in speed is not worth adding more weight and stress, and a larger motor will be disadvantageous to most.

I believe a 2.5 HP motor is the maximum anyone who is looking for a practical motor for the TI should select. This will give you decent speed and torque and not unduly overload the TI (or yourself) with excessive weight or stress. It will also give you optimum range. Having owned and tested a larger, heavier 3.5HP motor, I'm now very glad to be rid of it.

There will always be those who will push the limits of the TI, which is wonderful because it provides highly valuable information to the TI community, and these bold individuals usually understand and accept the risks and pitfalls. But those looking for safe, practical solutions would do well to stay within the reasonable limitations of what the TI is capable of.


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PostPosted: Fri Sep 22, 2017 9:23 am 
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pro10is wrote:
This is very useful information if it is accurate, and from my experience with TI motors it appears to be. It also clearly shows the declining justification of adding more weight and horsepower to achieve only modest gains in maximum speed. A slight gain in speed is not worth adding more weight and stress, and a larger motor will be disadvantageous to most.

I believe a 2.5 HP motor is the maximum anyone who is looking for a practical motor for the TI should select. This will give you decent speed and torque and not unduly overload the TI (or yourself) with excessive weight or stress. It will also give you optimum range. Having owned and tested a larger, heavier 3.5HP motor, I'm now very glad to be rid of it.

There will always be those who will push the limits of the TI, which is wonderful because it provides highly valuable information to the TI community, and these bold individuals usually understand and accept the risks and pitfalls. But those looking for safe, practical solutions would do well to stay within the reasonable limitations of what the TI is capable of.



Fully agree!! :)
As I stated earlier in the thread after my first testrun my personal recommendation for most users is a lighter motor 1,5-3hp. The next class of engines 4-6hp engines add at least an extra whopping 20lbs. The TI can handle the weight (I have no issues at all here that is with my heavy duty motormount) but the "Captain" (thats me by the way :lol: ) well I am having difficulties hauling the 50lbs from the car to the beach.
But you do get some extra speed (~30%) with the 5Hp and maybe a lower noise when not reving the motor as much, just using half trottle and still going 2-3 knots faster.
I still think I will have use of the extra "grunt" especially when my TI is fully loaded. It will be interesting to se how much speed is lost when adding another 200lbs of gear on my TI.
Maybe later on I can borrow a smaller outboard ( ~2,5 hp ) and make a comparison of both speed and milage with my heavy rig. That information could be interesting.
With a lighter setup I see no use whatsoever of a 5hp outboard unless for some reason you want to have that extra speed.
To bad I could not find a lighter 4-stroke (~3,5 hp) with external fuel line, reverse and exhaust tru the prop. I just dont like to compromise. :roll:
The extra exercise lifting my 5Hp will do me good!! (Or brake my back!) :shock: :lol:
I guess I can sell my gym-card now.... ehhh... no I cant... never owned one!!
:mrgreen:

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PostPosted: Fri Sep 22, 2017 10:02 am 
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Husse0416 wrote:
I am accually thinking about trying to tow a kayak after my TI for loading some gear and to use as a secondary kaya on longer trips.
I will test to tow my little china yak next season. I have heard that towing kayaks can be a bit problematic as they tend so zigzag a lot?
I have never tried it. Was thinking about rigging a rudder controlled by the towline if necessary to make the towed kayak follow better in line and nit zigzaging.

All the best!
/Gustav


Just set the rudder straight, no need to steer it.

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PostPosted: Fri Sep 22, 2017 10:53 am 
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This all towed stable.. The key was having the "center of drag" way to the rear. Two kayaks could store twice the stuff..

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