Tohatsu 9.8 Prop Pitch Question

[DECK SWABBER 58]

I have a 2014 Tohatsu 9.8 ultra long shaft 4-stroke which came with a high thrust 4 blade 8.7 x 5.0 prop.

Motor is on a 16' ft pontoon boat weighing approx. 1000 lbs. With two people and minimal gear the boat
goes 6 mph (cell phone gps) at 5750 rpm. I never expected this to be a high speed boat, I bought if for
small 10 hp limit lakes but a few more miles per hour would be nice for when we occasionally go on a no limit lake
just for safety's sake.

Tohatsu offers a same diameter 7 pitch prop as the only other option for this motor. My guess is the 7 pitch would
not make much of a difference. Are their any other options to get just a little more performance out of this set up?
This a remote motor, trim is set on #1 and cavitation plate is even with the keel.

 

[pvanv]

You are at a good WOT RPM. Could even go up to 6000. All the 8/9.8 props will interchange, but the HT props -- both 5 and 7 pitch -- are "stock" for HT (high thrust) setups. Going to the 7 pitch will slow your WOT RPM down a LOT... maybe around 4500, but probably the same hull speed... lugging if you will.

So, you are already about as good as you are going to get, short of having a prop shop slightly reduce pitch and add a bit of cup to the HT prop.

Ideally, you should have a much bigger motor, if the boat can handle it... we typically rig no smaller than 85% of the placard max hp.

All that said, a 3-blade prop might help a bit with max speed, but will likely slow your acceleration. See if your dealer will loan you one to try.

 

[Sea Rider]

Set trim to a hole that will achieve engine sitting perpendicular to toon, logo cowl must be parallel to water level when toon is floating all alone. Will have better prop angle of attack and prop thrust.. Why using a Ultra Long Shaft, is that a toon tech spec requirement ? You're good at 5750 wot rpm.

Happy Boating

 

[DECK SWABBER 58]

You are at a good WOT RPM. Could even go up to 6000. All the 8/9.8 props will interchange, but the HT props -- both 5 and 7 pitch -- are "stock" for HT (high thrust) setups. Going to the 7 pitch will slow your WOT RPM down a LOT... maybe around 4500, but probably the same hull speed... lugging if you will.

So, you are already about as good as you are going to get, short of having a prop shop slightly reduce pitch and add a bit of cup to the HT prop.

Ideally, you should have a much bigger motor, if the boat can handle it... we typically rig no smaller than 85% of the placard max hp.

All that said, a 3-blade prop might help a bit with max speed, but will likely slow your acceleration. See if your dealer will loan you one to try.

Funny when you know the answers to your post before posting! :lol:

We live 15 min. from 3 beautiful 10 hp limit lakes so that was what this boat was intended for. It has no rating plate (don't ask) but since I totally
rebuilt the transom which was originally about suitable for a trolling motor I'm sure it would easily handle a 20-30 hp motor. So I guess this old fart and his admiral will be quite content putt putting around our little scenic lakes at 6 mph.

Thanks, Paul

 

[DECK SWABBER 58]

Set trim to a hole that will achieve engine sitting perpendicular to toon, logo cowl must be parallel to water level when toon is floating all alone. Will have better prop angle of attack and prop thrust.. Why using a Ultra Long Shaft, is that a toon tech spec requirement ? You're good at 5750 wot rpm.

Happy Boating

Trim setting is exactly as you said. No, ultra long is not normal, if I knew then what I knew now I never would have bought this boat. :embarassed:
This is a home made boat my a mad scientist I found I online, not designed for a outboard motor. After rebuilding the weak transom
I put the short shaft from my jon boat on but it was practically under water, so I had to raise the transom so high I then had to go trade
the short shaft in for a ultra. Now it works, just slow. After fooling around with this for 4 yrs. it's all good, just slow. I'm done.

By chance are you in the market for a cheap pontoon boat so I can go by a "real one" ?