Propeller for 13-footer with Tohatsu 9.8

Optimizing the performance of Boston Whaler boats
f1loco
Posts: 23
Joined: Wed Jun 21, 2017 7:26 am

Propeller for 13-footer with Tohatsu 9.8

Postby f1loco » Mon Oct 16, 2017 2:05 pm

At present [my classic 13-foot Boston Whaler boat has] a Tohatsu 9.8-HP four-stroke-power-cycle short-shaft engine with a 8.5 x 7.5 propeller. I run a load of three or four adults. [The Tohatsu 9.8) is only [accelerating to] 5,000-RPM at WOT.

I am looking for a bit more RPM and possibly [for the hull to reach] plane. I know this is a lot to ask for a 9.8-HP.

The boat and engine feel like you are in fourth-gear but really should be in third-gear--if you know what I mean.

I put on a foil (to the anti-ventilation plate] hopes of easier planing, but no luck.

I see Tohatsu offers a 8.3 x 6 and a 8.0 x 5 three-blade propellers. I think the 2.5-inch drop in pitch to 5-inch would get me closer to 6,000-RPM at WOT and better [acceleration onto plane from a standing start], and the drop in diameter would keep slip resistance and strain down on the motor, compared to trying a four-blade propeller.

To anyone [with a 13-footer and a Tohatsu 9.8-HP engine and has tried]:
    --a 8.3 x 6 three-blade propeller
    --a 8.0 x 5 three-blade propeller
    --6-pitch proper

can your offer some feedback?

jimh
Posts: 11672
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
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Re: Propeller for 13-footer with Tohatsu 9.8

Postby jimh » Mon Oct 16, 2017 2:41 pm

In any small boat, particularly something as small as a 13-footer, the weight of the passengers becomes a very high percentage of the total boat weight, and particularly with four adults. If you want to get a 13-foot Boston Whaler on plane with four adults and a 9-HP engine, reduce the propeller pitch until the engine can accelerate to its maximum rated speed. If the boat planes, you have made it.

Because crew weight is such a high percentage of total weight, the performance of a small boat with limited horsepower will change quite significantly with added crew aboard.