Prop for 8 HP Tohatsu and 26' sailboat - SailNet Community
 
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post #1 of 7 Old 03-29-2011 Thread Starter
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Prop for 8 HP Tohatsu and 26' sailboat

I'm thinking of getting a high-thrust prop for my boat's outboard. The motor is a recent 4-stroke 8 HP Tohatsu and the boat is a San Juan 7.7, 26' LOA, 20' LWL and 3200 lbs design displacement. It has the standard 3 blade 8.5" x 7.5" pitch prop. Tohatsu has two high-thrust props available for the engine, 4 blade 8.7" in 5" and 7" pitch. Would either of these props be a better fit for my boat? And if so, which one?
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post #2 of 7 Old 03-29-2011
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The Diameter 8..26 x Pitch 5.9999 pitch worked well on my 4Hp pushing a 29' 8000# Cal 29 at 4.5 knots

I make a phone call to a place like InternetOutboards as the 5" may be a bit small

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post #3 of 7 Old 03-29-2011 Thread Starter
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My motor gets me up to speed fine. If my knotmeter can be believed I can just barely reach 7.00 kt under optimum conditions. Typical cruise speed would be around 6+ kt without towing a dinghy. I was hoping to get better control when docking and maybe better fuel economy with a high-thrust prop that hopefully slips less.
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post #4 of 7 Old 03-29-2011
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The HT prop pushes more because it allows the motor to reach its rated RPM at which point it makes 8 HP

You would need some type of handheld or tiny-tack to get and idea what RPM your reaching to make a good prop pick as i know even when pushing my J24 the stock prop was way to big and the HT one for the 4-6 HP series was massively better


I have never had a whole lot of luck with the stoping part using and outboard

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post #5 of 7 Old 03-29-2011
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I have an 8hp two stroke Yamaha for our inflatable. I changed out the standard 8.5"X 7.5 pitch for a 6.5 pitch and we still get up on a plane and can do over 15 knots, so I don't think the 5" pitch would be too small for you at all.

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post #6 of 7 Old 03-29-2011 Thread Starter
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I have a Faria outboard tach that I can connect to my motor, but I haven't mounted it yet. For some reason anything that requires me to drill holes in my boat takes a really long time to happen. I'd also like to think of some way to only power the tach when the outboard is running without needing to manually switch it on and off. The outboard doesn't have remote controls so there's no ignition key that would be a natural choice.

My boat is very similar to a J/24. Same LWL and draft and it's only 100 lbs heavier. About a 1.5' more freeboard and beam though. So the 8 HP motor is more than adequate to move the boat, even if the prop isn't the right fit. I could probably make do with a 4 or a 6, but the 8 is what the boat came with. And I like the electric start and alternator, which I don't think you can get on anyone's 4 HP and I don't think the 6 HP tohatsu has a real alternator. The two cylinder motor runs smoother too.

I guess the idea here is that less pitch means the motor will run at higher RPMs for a given throttle. So the goal is to get a prop with a small enough pitch to develop the max rated RPM at WOT, so you get the rated HP out of the engine. What's the issue with using a prop that has too little pitch? The motor will over rev? Or that one will lose top end speed? Is the speed loss possible for a sailboat or is that something only planing boat would have to worry about. I know if you calculate the pitch x the RPM at the prop you'll get a speed far faster than a displacement hull boat could possibly go.
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post #7 of 7 Old 03-29-2011
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You only need to use the tack to test your current setup to see if can be inproved as going to to small a prop would make it worse and give you the abilty to run the motor at to MUCH RPM

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