Inboard Issues

[mbd]

Yikes! Mike, thanks for digging it up. Would this coupler add to the length of the shaft and necessitate shortening my $$ brand new $$ shaft? Frankly, it sounds nice, but I'm thinking I should probably have a local yard take a look at the alignment and see if it is even an issue, and spend my dwindling boat funds on necessities right now.

Mike do you know the pitch of your current prop (and maybe the transmission ratio or type?)

Prop: 13x11
Transmission ratio: 2.62

I was able to borrow a prop puller, and the whole process was anti-climactic really. It just kind of came loose once I had tightened the nut down. After I cleaned off some of the bottom paint on the prop, I could read the above stamp.

Curious thing: after entering my data into a couple of prop calculators, they came back with a LARGER prop recommendation! Also, a very helpful guy at Michigan Wheel said that this prop is about the theoretical size for my given parameters.

The only thing I can think is the tachometer is malfunctioning. Is it possible to have a "wrong" tachometer that doesn't read the RPMs correctly?

Could it be something else? Am I missing something?

[Mike Goodwin]

You want to reach hull speed @80% of max rpm {1065.6rpm shaft}( for many reasons) and you want the boat to advance at the same rate as the prop .
So with a 13x11 prop at 1065.6 shaft rpms you are trying to push the boat at 11.1mph ( twice as fast as it should be ).

With a 13x11 prop turning at 1332 rpms (shaft) , you would be doing close to 15 mph or 14 knots + or - OR in order for you engine to max @ 3600 rpm with a reduction of 2.62 giving a shaft speed of 1332 rpms the boat would have to be going 14 to 15 mph .

I would say you need a 13x6 or 12x6 or 10x7 . You have too much diameter and too much pitch for 13.? HP which you only reach at 3600rpm , at 2100 it is much less. Your peak torque curve is near 2880 rpms ( engine ) and a 13x6 would put you in that ballpark .

Too much diameter is hard to turn over and too much pitch is just as bad.
I bet your rpm's drop radically when you put her in gear at idle , almost to a stall .

Will your tach read above 2100rpm when not in gear?

This is a 2 blade prop , right?

When you talk with anyone , make sure they understand it is 5000lbs you are trying to push to no more than 6 knots .

If your prop is too big and there is not enough 'tip' clearence , you will get vibration as the prop passes the hull . You need aprox. 18% dia. in clearence, 20% is better, so your tip should not come within 2.5" of the hull .

It is a big ballancing act to get the prop right .

[mbd]

It's a Yanmar 2GM. 13HP, 3400 RPM, and 15HP at 3600 RPM max.

I also have a three bladed prop. (See pictures)

"Will your tach read above 2100rpm when not in gear?"

This is the million dollar question for which I am ashamed to admit, I have no answer.

I didn't realize I was in trouble until after the season and I was reading the manual while winterizing the engine. I'm pretty sure it does - at least the engine revs pretty good when in neutral. I just never looked at the tachometer while doing it.

[Mike Goodwin]

You have one blade too many on that prop . No way you can turn a 13x11- 3 blade with 13 hp unless it was in a 400lb skiff . At 2100 rpms your are not even getting 13hp. That is a huge prop for a small boat .

My figures were for a 2 blade 13x11 , that extra blade needs more hp to spin it.

It is probably a motor boat prop too , and has huge surface area .

Get a 12x6 sailor prop (2 blade ) and you will be happy for the rest of your life .
You will sail faster too (less drag )
1216 shaft rpms should give you 6+or- knots with a 12x6 ( 6" pitch ) prop , so if you throttle back a little 5.5 knots is easy .
You know you can have props re-pitched , so get a 2 blade that is close and try it and if it is not exact either pitch it up or down until you can get 5.5knots at 2800 rpms .
There are a lot of other factors involved , but this gets you in the ball park .
I have re-proped dozens of boats , half were right on the 1st try and the others took one more pitch change to get close enough . planing motorboats are a lot harder to get right .

[Mike Goodwin]

BTW , the Vetus coupling doesn't add any length to the drive train .

[mbd]

Mike, thanks as always!

BTW , the Vetus coupling doesn't add any length to the drive train .

I'll get back to you in the spring on this one.

At 2100 rpms your are not even getting 13hp. That is a huge prop for a small boat.

This is where I started. My local boatyard said the same thing and I was planning on downsizing until I ran the figures through the prop calculators.

I suppose one route to go, after rewiring the tachometer, is to find a smaller cheap prop somewhere and see how that does next season. I'd hate to muck it up so bad that it'd force a haulout to replace the prop - the season is short enough as it is.

Thanks again.

[mbd]

BTW, I'd really like to hear from other Ariel/Commander/Triton inboard owners what their setups are. (Engine HP/RPM, transmission ratio, prop size/pitch)

Any info appreciated!