Which prop for my Tohatsu M3.5B2 on a small sailboat (650 kg)

[daveson]

Hello,

i have a small sailboat which weighs about 650kg including crew and gear and has 5.5m waterline. I use a Tohatsu M3.5B2 with the normal 7" pitch propeller.
I think it is a little bit too much power, the transom is only made for 2.5 hp said the manufacturer of the boat. Thats why i always use only 1/2 or 3/4 of the power. The boat reaches almost hull speed with this low setting.

The only problem is, that the shear pins broke quite often while in waves, because the prop went out of the water shortly and then the shear pin broke.
I want to buy a aluminium prop with the better stainless steel shear pins now. Do you think this would help?

The only question is, if i buy a new prop, which is the correct size? The one i already have with pitch 7? Or the same diameter but pitch 6? And then there is one with bigger diameter (288mm instead of 188mm) but with picht 4.5 only (314-64103-0). I think this prop is often used for sailboats, but perhaps for larger ones which need this extra power? Or would it be good for my boat too?

Heres what i expect from the engine:
- Hull speed with as low noise as possible (does this mean higher pitch, lower rpm?)
- power for windy conditions or to go against the current in a river

Thanks for your help,
daveson

 

[Sea Rider]

Re: Which prop for my Tohatsu M3.5B2 on a small sailboat (650 kg)

Tohatsu 2.5 & 3.5 are same bothers, being the first carb restricted. Could try a 6" pitch alum prop with ss shear pin. Works betetr than plastic props and their shear pins.

Happy Boating

 

[TOHATSU GURU]

Re: Which prop for my Tohatsu M3.5B2 on a small sailboat (650 kg)

The shear pins are now SS for both plastic and aluminum props. Diameter is unimportant as you have no choice in diameter within the same pitch. Just go with the lowest pitch available and live with the increased engine RPM and noise.

 

[daveson]

Re: Which prop for my Tohatsu M3.5B2 on a small sailboat (650 kg)

The shear pins are now SS for both plastic and aluminum props. Diameter is unimportant as you have no choice in diameter within the same pitch. Just go with the lowest pitch available and live with the increased engine RPM and noise.

Thanks for your answers. Just for better understanding, what would be the advantages of a lower pitch propeller for my boat? I thought lower pitch is only needed for heavy boats with a weak engine. Or is my boat already that heavy that lower pitch is better?

Thanks
Daveson

 

[steelespike]

Re: Which prop for my Tohatsu M3.5B2 on a small sailboat (650 kg)

The important thing is that the motor be able to rev freely within its designed range.
A 6 will allow easier reving.If you think about it a 3.5 is pretty much overwhelmed with a boat of a size its intended for let alone a boat over 1,000lbs.

 

[TOHATSU GURU]

Re: Which prop for my Tohatsu M3.5B2 on a small sailboat (650 kg)

Your boat is heavy enough that the lower pitch would provide more power in a headwind or current...That would translate as being more fuel efficient in your application.

 

[Sea Rider]

Re: Which prop for my Tohatsu M3.5B2 on a small sailboat (650 kg)

If your sailboat weights less than a J-24, stay with actual prop, just change shear pin for a stainless one if having issues with standard ones. Will need to make a wot test with a tacho and test both props to see if achieving higher rpm's, between 4,200 to max 5,200 rpm as recommended by the factory. Anyway in calm water conditions, no sailboat runs at wot all the time, 1/2 to 3/4 throttle is ok.

Happy Boating

 

[pvanv]

Re: Which prop for my Tohatsu M3.5B2 on a small sailboat (650 kg)

All the notes that Steel, Elvin, and Luis have mentioned are correct.

Run the ss pins. That's all we carry now anyway.

As for pitch on a sail application, go as low as you can without exceeding the factory-recommended WOT RPM. Having the motor in the middle of the WOT RPM range develops maximum torque, which translates into greatest efficiency as well. The only way to confirm that is with a tach. According to the owner's manual, you want to be somewhere between 4200 and 5300. 5000 would be excellent.

Some other notes: On sail applications (slow boat, heavy load, pushed by a small motor), small pitch changes make a very big difference in RPM. Is this a long shaft motor? If not, consider swapping for the long version in order to help keep the prop in the water, and make your impeller last longer. As for the rating of the mount (2.5 hp vs the 3.5 motor), you would know if you overpowered the bracket/transom, and you would know very quickly.

 

[daveson]

Re: Which prop for my Tohatsu M3.5B2 on a small sailboat (650 kg)

Ok, the boat on its own weighs only 450kg, but with me+1 and all the equipment i thought it would be around 650 kg. Its more like a sailing dinghy, an open keel boat. A lot less than a J24 for sure.
In fact i NEVER even tried to put the engine to maximum rpm because i was afraid it will overrev. It becomes very loud even at 3/4. But perhaps i'm just to frightened. Thats why i'm not sure about the lower 6 pitch propeller.
But how can i measure the rpm? They are not being displayed like in my car... ;-)

 

[steelespike]

Re: Which prop for my Tohatsu M3.5B2 on a small sailboat (650 kg)

For easy multiplication isn't it about 2lbs per kg. that would put just the boat at about 900 lbs.
Assuming the prop isn't venting it isn't going to overrev pushing something like 1300 lbs.total weight.If it is venting you would be reving with no speed increase. Might actually lose speed.
You can check your rpm with a tiny tach,about $50 and easy to install.They have a web site,be sure to read all info.

 

[daveson]

Re: Which prop for my Tohatsu M3.5B2 on a small sailboat (650 kg)

Ok, i'll start measuring the rpm when cruising at different power settings and then i'll decide which prop to buy. But you're probably right. I found a Tohatsu dealer page on the internet where it says that everything from 0-500 lbs is light load, 500-1500 moderate and above heavy load. The 7inch pitch prop is for light load only and my boat weighs already 1000lbs when empty, so i'll have to choose lower pitch prop.

Thanks for your help!

 

[steelespike]

Re: Which prop for my Tohatsu M3.5B2 on a small sailboat (650 kg)

Let us know how you make out.Its allways nice to get additional real life results.

 

[Sea Rider]

Re: Which prop for my Tohatsu M3.5B2 on a small sailboat (650 kg)

Is not a commercial, go for the Hardline tach/hour instead, only $ 43.00 shipping included, very nice and friendly to install. Hour and Tach/Hour meters cost the same, go for the second better alternative at same price. Start with actual prop and check max rpm at wot, then decide for the 6" or lower pitch prop.

http://www.hardlineproducts.com/showpage.cfm?catid=136 (Model HR806)


Happy Boating