What kind of prop for my situation?

[kooker]

I just picked up a mid 90's Tohatsu M40c. It's on a 15' semi-v flats boat that according to the manufacture weighs roughly 550lbs. I'm going to be using it mostly in the flats so getting up quick in shallow water is ideal. The boats max HP is 40 so I'm sure with the HP i'll get some speed but I'm not overly worried about that. I'm more concerned with holeshot and getting on plane quickly. Currently I don't have a jack plate but I may get one or a fixed jack plate soon. With all being said, what pitch or type of prop do you think I should get?

 

[gm280]

First of all kooker, welcome to iboats :welcome: good to have you join us.

As for the prop. Just remember the lower the prop pitch the better the hole shot. So depending on what you presently have, go down a size or two and you will get your hole shot you're wanting. But as usual, nothing comes without other issues to think about. That being the lower the prop pitch to give a better hole shot, the more the upper end RPMs are going to rive. And that can lead to some serious over-riving your engine and the possibility for engine damaging it for certain. So if you're going for a quick hole shot, be extremely careful with WOT RPMs...

 

[kooker]

I have a tiny tach that I'm planning on installing so I'll have a better understanding of RPM's, but at the same time, I'm not planning on WOT due to having so much horsepower. I'll most likely always be 3/4 throttle. Don't 4 blade props generally get better hole shot than 3 blade? If that's the case, I could go with a 4 blade and keep the pitch the same?

 

[steelespike]

We need wot rpm and gps speed to evaluate your present setup.Need the prop size (under the prop nut or near the base of the blades.
If you read the stickys at the top of the prop forum, it may help you understand prop selection better.

 

[kooker]

Haven't used the engine yet. I'm attending to a possible seal leak in the LU right now. Here's a picture of the only information I found on the prop. I'm guessing it means 10" diameter, 3 blade, 9 pitch? I honestly have no clue. Ideally I'd like a stainless prop. This is obviously an aluminum

 

[steelespike]

No numbers under the prop nut?
The boat is no fly weight.I have a 18' Cresliner bare utility that weighs 375lbs.
The actual size of your prop would depend on the typical load and activity.
If you look up the Tohatsu prop chart you will note the weights over lap
in the load chart and there is no consideration of hull design.
There are at least 2 warnings not to over rev the motor. You will need a tach and a gps to
evaluate your setup.

 

[Sea Rider]

Why does new boaters likes to ask how to obtain best prop advice if they haven't tested their current one. If you plan using that combo lightly loaded or with few boating mates, that light boat with a 40 HP should fly direct to Mars.

Standar factory delivered props on light to medium applications gives excellent hole shot and good top speed. Just throttle accordingly your boating requirements for more or less speed.

Some issue about Tohatsu OB's, likes being vertical trimmed and spot on transom height seated. Works best with water flow skimming right under small upper water defector plate (yellow line) as opposed to middle or near anticav plate for best prop thrust and overall combo water performance including tight close turns at any water cond.




In order for a Tiny Tach to read correct rpm, must be set to 180? usually comes factory set to 360? at that settting will read double rpm. That engine wot revs at 5200-5800 rpm. What you need to do is place a mate up front at bow, go for a wot spin on flat glassy non wind water cond , check if having out or over transom water splashes and max WOT rpm achieved. If inside middle to max (5500-5800 rpm) stay with that prop, if over revving slightly throttle less. You can install that tach at front of cowl, next to tiller to monitor engine revs at all times.

Post current wot rpm and GPS speed.

Happy Boating

 

[kooker]

Changed the gear lube and ran it today with the prop above. With myself and another 220lb guy, we were getting 35 mph and could probably get a couple more mph out of it.

 

[Sea Rider]

Before hoping for more speed you should tach that engine as you don't know if engine is working at its stated factory wot min-max rpm parameters with current prop.

Happy Boating

 

[kooker]

Before hoping for more speed you should tach that engine as you don't know if engine is working at its stated factory wot min-max rpm parameters with current prop.

Happy Boating

Honestly, I don't need to go any faster. I'm happy with the speed as of right now. I'd rather just find a good stainless prop and leave it at that

 

[steelespike]

You don't know your rpm without the rpm and gps speed we don't even know if we should go up or down with our prop selection.
Most cells have a gps app.And a induction tach will get you your rpm.
Have you looked under the prop nut for a prop size? With the motor setup right; hole shot will be better and the responce to added weight will be better.
The motopr will be happier.
The prop selection guide has 2 warnings in red to not exceed max. rpm.
A well setup motor will likely be faster but you can operate at what ever speed you prefer
and you will have a motor at its most efficient operation.

 

[Sea Rider]

If you had a larger boat & engine you definitely will have a tach on dashboard to count with. On portable engines you need to check current max rpm for engine to work as was designed to do so. Steelespike describes very well your actual situation. Invest $ 40.00 wise on a Tiny Tach, Hardline induction tach and play with alum, SS and prop pitches all you want but safely. Your engine will be eternally grateful...

Happy Boating.