Suzuki/Tohatsu High Thrust

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jbo_c
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Suzuki/Tohatsu High Thrust

Post by jbo_c »

It seems Suzuki(and also Tohatsu?) doesn’t have a differently geared high thrust. You just put a four blade prop - different pitch, of course - on rather than a three blade.

I’m thinking a 20hp high thrust for my shanty. Projected at 3500 pounds cruising weight and a flat bottom. Will this setup work? Enough prop options? Or do I need to go with another manufacturer that has a lower unit that’s actually geared differently for high thrust?

Thanks.

Jbo

bklake
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Re: Suzuki/Tohatsu High Thrust

Post by bklake »

Sailboat people would be a good source of info on this. A 24-26ft sailboat weighs in that range. I'm pretty sure they get by just fine on 9.9hp but they have less drag in the water. 20hp is clearly better just because it is double the HP and the same weight. Never know when you are going to be pushing against a strong wind.

My Suzuki 9.9 came with a 5" pitch 4 blade high thrust prop. I used it for the first 5 hours while I had another prop on order. My boat weighs under 500lbs. At max RPM, the boat was going a blistering 8 MPH. The motor had no problem reving to max RPM almost instantly. In fact it was very difficult to control the RPMs with little load on the engine.

I wouldn't get too wrapped up with the gear ratio. The motor can turn the prop. Solas makes a full line of 3 and 4 blade props. No worries there.

bklake
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Re: Suzuki/Tohatsu High Thrust

Post by bklake »

Suzuki gear ratio is 2.08:1
Tohatsu is 2.15:1
Honda is 2.08:1 in 20hp, 2.33:1 in 9.9 Carburetor engine, not for me.

Tohatsu and Honda won't sell remote engines directly, have to go to a dealer if you want a remote. Suzuki will sell a remote model directly. I went through a dealer but they put the box in my truck and wished me luck. I did not have to have them install it. Personal choice on that one. I was comfortable doing the installation.

jbo_c
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Re: Suzuki/Tohatsu High Thrust

Post by jbo_c »

Thanks, BB. Yes. Planning to self install.

Jbo

Cowbro
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Re: Suzuki/Tohatsu High Thrust

Post by Cowbro »

I have the 9.9 EFI suzuki with a 4 blade prop on my HMD 19, it works great for that purpose. the Suzuki 9.9/15/20 are all the same with just different computers/air intake/restrictor plates.

If you can handle the price increase and a couple extra pounds i would go for the suzuki 25 for the larger gearbox and prop diameter (i think it is only like 25-30 lbs extra. I think the little 9.9/20 is going to be out numbered on windy days when you need some grunt to keep control of the shanty.

Phil

fallguy1000
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Re: Suzuki/Tohatsu High Thrust

Post by fallguy1000 »

Compare all the engines for prop capacity. You want a big prop for that boat.
My boat build is here -------->

viewtopic.php?f=12&t=62495

TomW1
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Re: Suzuki/Tohatsu High Thrust

Post by TomW1 »

jbo stick with the Suzuki the gear ratio of the high-thrust Yamaha is the same as the Suzuki at 2.08. No use spending the extra $$. Let me know if I can help you in any way size your motor. I agree with fallguy also.

Tom
Restored Mirror Dinghy, Bought OD18 built by CL, Westlawn School of Yacht Design courses. LT US Navy 1970-1978

jbo_c
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Re: Suzuki/Tohatsu High Thrust

Post by jbo_c »

Thanks, guys.

Tom, your advice early on in my build thread was that a 9.9 high thrust would be OK, but a 15 horse would give me a little reserve. That’s largely what I’m basing my decision for the 20hp on.

That’s the highest horsepower in that family without moving up in weight if I’m right - thus, max ‘reserve’ in the weight class. My design is already a little stern heavy at rest, so don’t want to add any weight I don’t have to back there.

Jbo

bklake
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Re: Suzuki/Tohatsu High Thrust

Post by bklake »

jbo_c wrote: Mon Feb 12, 2024 10:06 am Thanks, BB. Yes. Planning to self install.

Jbo
I typed in "Suzuki install guide" or something similar and the actual Suzuki manual came up. It is detailed, lays out the steps in order and gives some measurements you need to check. I thought their order didn't make sense but followed it. It did make sense and I would have had to remove stuff to complete other stuff if I had not followed the steps. It's about an hour of work to get the cables and harness rigged.

If you can take the weight of the 25, I would go that route. It's only a few hundred more. I'm sure my 9.9/20 will live a long happy life but at 20HP it is putting out 99.9% effort, stressed to near it's limit. The 25HP has more mass, more CCs, more in reserve. Cruising 5kts is going to take the same HP. The fuel burn per HP is probably within a percent or two between the engines. At the top of the power band, the 25 can burn a lot more. How much time will be spent at max power?

It cost a couple hundred $ more to convert a 9.9 to 20. People do it because of restricted HP waterways or other regulation work arounds. You have a motor with a 9.9 VIN and 9.9 stickers on it. Kind of difficult to prove it isn't a 9.9 unless you dig the computer out and match part numbers. I told the dealer what I was doing. He said it was not possible, it would void the warranty. I said I don't really care about the warranty. If it runs fine through break-in, it will probably be fine. After a few rounds of this and convinced I knew what I was doing, he reach under the counter and pulled out his HP conversion "kit". Computer, restrictor, and some other small part I needed. He advised me to break it in on the 9.9 computer, keep that computer on the shelf, and if there was a problem, swap it back in before bringing it in. Nobody will know. I did it so my kids could operate the boat, no papers needed if less than 10HP. They are old enough for a boater license now so I converted it.

jbo_c
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Re: Suzuki/Tohatsu High Thrust

Post by jbo_c »

I’m guessing 5% or less of the time will be above half throttle. I’m guessing most at little more than idle- call it 25%. My wife and I are pokey people. I had a center console fishing boat for years and it probably was run on plane less than an hour over a decade.

The only time we’d be above half throttle would be if necessary to overcome wind or current and between the plan of use and proper planning, that would only be if we were caught off-guard. Good conservative planning should make that time negligible and short-lived even when it does happen. If it’s too bad, we either won’t be out or will duck into a pocket and just wait it out.

Jbo

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