Tohatsu 9.8 high torque question

[Yooks]

Hello. I'm building an 11' garvey-style skiff. The boat will weigh around 100lbs and have a 10 degree deadrise. Based on user experience, it sounds like a 10 hp outboard will plane the boat with two adults and one small child. I'd prefer to be able to plane with a slightly larger load (2 adults and 2 medium size kids.)

I'm looking at the Tohatsu 9.8 with a 20" shaft. Will the "high thrust" prop (as i understand it there are two different 4-blade props, a 7 pitch and a 5 pitch) on a tohatsu 9.8 help get the boat up on plane? I'm not too worried about sacrificing top end speed as my sons will some day be driving the boat and I don't want it to go too fast with a light load anyway. I just want to get out of the hole and on plane.

Alternatively, would you suggest I move down in pitch with a standard 3 blade prop to a 6 pitch?

I'd prefer to stick with the 9.8 rather than move up to the 15 due to weight considerations.

Can anyone tell me what the speed difference might be between the standard prop and the ht?

Thanks,
j

 

[pvanv]

Re: Tohatsu 9.8 high torque question

The HT props are typically specified for heavy displacement, such as a sailboat or barge. They have however been used successfully on RIB setups as well. They are intended for slow speeds. They do shovel a lot of water at slow speeds. If you want to try one on such a light boat, I would try the 7 pitch. I have a 5 pitch on my sailboat, and achieve 6,000 RPM at 5 knots. Obviously there is some slippage, but I would expect your light boat would over-rev with the 5 pitch.

 

[kfa4303]

Re: Tohatsu 9.8 high torque question

Hi yooks. Welcome to iboats. Just curious why you're going with a 20", longshaft motor on such a small boat? If anything, I would get the 15" standard/shortshaft and put it on a jackplate. Before you start your build, have you checked out a boat called a Gheenoe? They're made down here in FL where they have sort of a cult following. They are essentially unsinkable, hot-rod canoes that can hold upto 3 people and a 40 hp OB as well as being able to plane out in under 6" of water. Just something to think about before you start your build. You may also want to check out microskiff.com for lots of great ideas for small boats.

http://www.youtube.com/watch?v=0cTLSfUMQ2Y

http://www.youtube.com/watch?v=xqcEkibszt0&feature=related

http://www.youtube.com/watch?v=e8vOjs6nhu4&feature=related

 

[Yooks]

Re: Tohatsu 9.8 high torque question

Thanks for the replies everyone, and for the welcome.

I have seen Gheenoes; they are impressive.

But I would like to tow my skiff behind a sailboat, so I'm leaning towards the GV11: http://www.bateau.com/proddetail.php?prod=GV11

Based on the experiences of previous builders, it sounds like this boat is happiest with 15 Hp. That hp will plane two adults and two kids. The specified hp, 10, struggles to get 2 adults a 1 kid on plane. A 5 hp is supposed to get 1 adult on plane.

I need to keep the weight down since I'll be towing the skiff behind my sailboat. It would be a bad idea to do that with a 150 lb ob on the transom.

So my thought was to sort of gear down a a smaller outboard to get larger loads on plane. I'm happy to sacrifice top end as a result.

I have a 5 hp 2-stroke Tohatsu that I will try first. If I can get the skiff to a reasonably efficient plane with a couple kids and an adult then I'l call it good enough.

Next option would be a light and quiet 4 stroke like the Tohatsu 9.8.

Final option would be something like the Zuk 15, which comes in at ariund 100 lbs.

A wild card option would be an old Evinrude/Johnson 9.9/15 2-stroke, which comes in at around 80 lbs, but I imagine they are fairly loud. Great engines though, from what I've heard.

As for the transom height, I've heard that a short transom tends to ship water over the stern when coming off plane. But a jack plate is a good idea. I like the idea of tuning the height of the cavitation plate. Any suggestions for a light weight jack plate?

Thanks,
j