What size motor ?

[thomm225]

Originally Posted by flynnboy

Thanks searenity38. Is 20" a longshaft ?

20" is a longshaft; 25" is extra long shaft. (15" is short shaft)

I have the 25" on an adjustable bracket (11" throw) to push my 6600 lb disp Bristol 27.

I'm on my second bracket now. Motor is a 2011 bought new then. (second tiller also)

[Chotu]

Originally Posted by sanibel sailor

I guess the law of equal and opposite reaction does not apply to smaller props?

It does, but you’re neglecting mass when thinking it through.

What’s the mass of a 12” diameter column of water?

What’s the mass of a 4” diameter column of water?

Which one is going to have more mass to push the boat in the equal an opposite direction?

The small prop is pushing against a lightweight column of water that’s slipping through other stationary water. The large diameter prop is pushing against a big, heavy column of water that also has (simply put) more wetted surface area to create friction with the stationary water.


And speaking of the water column’s slip, that makes sense too.

A tiny 1” column of water has a circumference of about 1.25 inches. That’s the amount of water in a given spot providing the friction with the other stationary water to hold that column in place.

A 12” column of water has a 14.8 inch circumference.

It’s analogous to a tire tread.

Do you get more grip from 1.25 square inches of tire sticking to the road or 14.8 square inches of tire sticking to the road?

How about you dump 100hp into that tire with a tiny 1.25” patch on the road vs dumping 100hp into a tire with 14.8” patch touching the road?

What happens in each case?

Same as with props.

[drdoyle]

Originally Posted by sanibel sailor

I guess the law of equal and opposite reaction does not apply to smaller props?

No it doesn't apply because the ability of the smaller diameter prop to push the water is considerably less efficient in converting a given horse power input to thrust. The smaller diameter has greater slippage and other deficiencies as its airfoil shaped short blades have more tip losses. If you take the same engine and change from a smaller diameter higher pitch prop to a larger diameter lower pitch prop the efficiency goes up dramatically. If you then get the same engine with a steeper gear reduction and turn the prop slower you can increase the prop diameter even more. Slower turning larger diameter props are much more efficient in producing thrust in a displacement vessel than smaller diameter props.

[E26-2]

Originally Posted by Compass790

+1 for this post.
With that weight boat 6hp will be ok. We have a 26ft boat weighing at a guess 1.5 tonne & an 8hp 2stroke tohatsu outboard drove it pretty well when its diesel engine was inoperable.

I have a 7.5 tanner and use a 6 h.p. 2 stroke. We have a 5 knot tide and it pushes the boat fine.

[Icarus]

Originally Posted by flynnboy

Hi chotu,

Thanks very much. The truth is that we live in France and whereas I would definitely prefer a bigger motor if I have anything bigger than 6 hp I am obliged to do a course to get a permit. I have no particular objection to that either, my only problem is that I am not sure that my French language skills will get me through the course and it costs between 300 and 500 Euro. That is not money I can afford to throw away and so we were hoping that a 6 hp would do the job.

We intend doing inshore fishing off the coast, within sight of land and would not be venturing out in bad weather. I have attached a picture.

Thanks again for your advice.

Get a 9.9hp and change the decal sticker to 6hp

[Chotu]

Originally Posted by Icarus

Get a 9.9hp and change the decal sticker to 6hp

This!!!!! Ha ha ha.

Best idea on here.

[drdoyle]

Originally Posted by Icarus

Get a 9.9hp and change the decal sticker to 6hp

Definitely yes. We have the same type restrictions in some USA states. 9.9hp no registration required, 10hp up registration required. Many take the 9.9 (which is a de-tuned 15hp motor and remove the carburetor restrictions. And some get a 15hp and change the decals to 9.9hp

[drdoyle]

in any case get the largest diameter propeller that fits into the clear diameter of the lower case, then reduce the pitch number to the lowest available. This will double the efficiency of the thrust in a 6 knot boat.