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I plan to repower a NAUSET [presumed to be a 16] with a Tohatsu 60-HP four-stroke-power-cycle engine with tiller steering.

Q1: what propeller pitch will produce a trolling speed of 2-MPH or less at engine idle speed?

My plan is to select a propeller that will "meet" the engine RPM limit, but not less.


BACKSTORY
I bought this 1964 NAUSET 16 whose interior was in rough shape, but the hull condition was "decent." I plan to completely gut [the interior], then have only a fuel tank, a battery, and two seats. This boat will be used for angling, and mostly for the species Salmon. Occasionally I will use the boat on a lake. I do not care about the maximum boat speed.

For readers who have experience in re-powering [a stripped out 16-foot Boston Whaler hull] with a 60-HP [Tohatsu four-stroke-power-cycle engine with tiller steering], [please comment].

[Moderator's note: this thread is moved to the PERFORMANCE forum for discussion from the REPAIRS and MODIFICATIONS forum. Predictions of boat speed following a re-powering with a different engine are always discussed in the PERFORMANCE forum.]

Ringo wrote:Q1: what propeller pitch will produce a trolling speed of 2-MPH or less at engine idle speed?

To answer your question regarding a prediction of boat speed produced by a propeller, the GEAR RATIO and engine IDLE SPEED RPM must be specified. Please provide that information.

Once that information is available, the BOAT SPEED in MPH can be calculated with the PROPELLER CALCULATOR available on this website at

https://continuouswave.com/calculators/propCalc.php

Generally the performance of a propeller operating at very slow boat speed will exhibit a characteristic of SLIP in the range of 30 or possibly higher. Because of the uncertainty of the value for SLIP, predicting the exact boat speed will be difficult.

You should proceed to the calculator, enter the RPM and RATIO data, and set the boat speed (MPH) to the desired value (2-MPH). Set the SLIP to be 30, and the calculator will deduce the propeller PITCH.

Ringo wrote:My plan is to select a propeller that will "meet" the engine RPM limit, but not less.

This comment is somewhat unclear. My interpretation is you will choose the propeller so that at full-throttle setting the engine will accelerate to the maximum allowed engine speed. This method of propeller selection may not allow you to achieve your other criterion of reaching a boat speed of 2-MPH or less at engine idle speed.

To achieve a particular boat speed needed for angling using the trolling method, two techniques are often employed:

• a mechanical device is used to spoil the propeller thrust, sometimes in the form of a hinged plate that attaches to the engine leg at the anti-ventilation plate and is lowed into position aft of the propeller to disrupt the stream of accelerated water coming off the propeller; or
• the engine is operated in reverse and the boat is navigated transom-first instead of bow-first.

Thank you for that. All very good information!

Regarding a Tohatsu 60-HP engine, there are three distinct models available:

• the model MFS60A, manufactured by Tohatsu; and
• models BFT60A and BFW60A, made by HONDA and re-badged as Tohatsu.

The specifics of gear ratio and engine speed ranges are probably not identical between all models of 60-HP Tohatsu engines.

This information was just recently educed in a concurrent discussion.

Cf.: https://continuouswave.com/forum/viewtopic.php?f=7&t=8746#p50442

Assuming the following inputs to the PROPELLER CALCULATOR

RPM = 500
RATIO = 2.08
PITCH = (to be calculated)
SLIP = 30 (a reasonable value for propeller SLIP at really low speed operation)
MPH = 2

The calculated propeller pitch needed would be 12.6-inches.

Now using a 12-pitch propeller we can estimate the maximum boat speed

RPM = 5500 (assumed value for the engine)
RATIO = 2.08
PITCH = 12 (about as calculated above)
SLIP = 10 (typical at speed
MPH = (to be calculated)

The calculated boat speed is 27-MPH.

Cf.: https://continuouswave.com/calculators/propCalc.php