Tohatsu propeller choice

[Greenheart]

This is an idle enquiry about a general point, not specific to my own boat or purposes.

Tohatsu outboards seem to be available with different sizes of propeller, to suit a range of applications.

The 20hp can turn a maximum 10" prop...while the slightly more powerful, 25hp 3-cyl is available with a big 14" prop.

The latter sounds big enough to drive a substantial displacement hull...

...but you will probably have noticed in all photos of outboards, that the propeller blades' outermost 'turning circle' passes extremely close to the horizontal 'fin' protruding from the shaft just above the prop...

...so, assuming the photograph is not of the very largest propeller, what modification of the shaft must be undertaken, to make room for an extra two or three-inch radius of prop? (I'm not asking about adjustments to internal gearing, which the manufacturers must recommend for non-standard prop sizes.)

I don't know what the little fin or tab is called - nor how critical it is to the engine's performance. Any comments? Thanks.

Here's the page where I found the data: http://www.tohatsu.com/outboards/25_4st_spec.html

 

[VicS]

The maximum prop diameter is limited by the distance between the prop shaft and the anti-ventilation (AV) plate ( often incorrectly called the anti-cavitation plate)

There are no mods or alternatives to the gearcase that allow larger diameter props to be fitted.

The full prop spec should include the diameter and pitch ( normally quoted as diameter x pitch )
The figures you see are almost certainly different pitch sizes. For some reason Tohatsu do not seem to quote the diameters. Small pitch for slow displacement speeds. Larger pitch for higher speeds.

AFAIK there is no variation in internal gearing. It is not necessary as the effect is achieved by the different prop pitches

Props are also available in aluminium or stainless steel.


The little vertical fin behind the prop is the trim tab. It is adjustable, to compensate for the paddle wheel effect of the prop, so that the boat will run in a straight line when the engine is straight rather than tending to turn in a circle.


For prop details see http://www.tohatsu.com/accessories/prop.asp

 

[Greenheart]

Thanks for that Vic - I'm smarter now than when I woke up.

I should have guessed that changing prop-pitch is a much easier way of effectively reducing the final drive - but if that is effective, why were some "saildrive" outboards available with different gear ratios, to create more thrust than just high-revving "squirt"?

 

[VicS]

No idea.

 

[Greenheart]

Thanks again - the prop-details page clarifies much about pitch, which I'd never understood before.

I had been somewhat surprised by Dylan Winter's success, driving the Centaur with a 6hp outboard. But if the propeller is pitched to thrust itself steadily through the water rather than just frothing it up, I suppose there's no good reason why horsepower delivered by a long-shaft outboard should be any less effective for a small yacht, than an inboard. :encouragement:

 

[catalac08]

Outboards work quite well for cats-mine is an Ultra long Tohatsu 20HP and it never comes out except when it it very rough but by then I would want to slow down rather than bad slamming damaging the boat.