Mercruiser 120

We finally got the boat out yesterday for the maiden voayage. The outdrive had a whaletail and turnbuckle adjustable trimtabs on it when I bought it. As soon as I brought it home I turfed the whale tale and made sure the trim tabs were set even and they were set down to about 3/4 inch below the hull.

Specs. 72 Starcraft 18' openbow approx 1800lb, Mercruiser 120 1.98 GR, Mercury prop 15 1/4 X 21 GPS speed 33.9 mph at 3900 RPM

The prop on it is in real bad shape bent and chewed up. When we first hit the water I could not get the bow to come up, the trim tabs were so effective at 20 mph the prop would blow out and I would have to back off. I adjusted the trim tabs twice till they are almost parallel with the hull and things got way better, but it takes very little up trim for the prop to ventalate and blowout.
What recomendations do you have for this problem and props. I ordered a Solas 15,3/10 X 21 from I boats but it will not fit and I am trying to return it. Would a Hustler 14,1/4 X 21 or 23 work?
Any help would be appreciated.
Thanks Glen

Re: Mercruiser 120

It difficult to make a prop judgement without some solid numbers from a good prop.
You may be able to get the prop repaired and get some meaningful numbers.
Usually a damaged prop will have a lot of slip though it depends on the damage.
I would guess the 21 could be selected for general cruising and top speed. Probably not much good for skiing and such.I assume your max rpm should be something like 4200-4400.A label may be on the motor.A new 19" prop would add about 300 to 400 rpm and probably trim out better.And would have a little better hole shot.You might back off the tabs for a couple of high speed runs.And then add them a little at a time.Too much tab will not only put more boat in the water but cause drag themselves.

Re: Mercruiser 120

Thanks Steelespike. After looking at the prop I have, and going over what happened, I am thinking that the hub is slipping in this prop. Would this only happen when it gets to it highest load as in when trimming out?. The manual states WOT to be between 3900 and 4300 rpm. How much difference would going to a 14,1/4" diameter make from the 15, 1/4?
Thanks again for you help.
Glen

Re: Mercruiser 120

dozerII said:

The prop on it is in real bad shape bent and chewed up.

the prop would blow out and I would have to back off.
Thanks Glen

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You have to have a prop in good condition to make a judgement. A poor prop will not give you any info at all.

With the 1.98 ratio the 21 sounds like a good pitch.
But dont go dropping a ton of coin on a SS prop until you know what you need. Stainless is very expensive to buy and they are not worth anything for resale.

Have your redone at a prop shop for about 60 bucks then go from there.

Re: Mercruiser 120

I ordered a Solas 15,3/10 X 21

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Why wont this fit?

Re: Mercruiser 120

bifflefan said:

Why wont this fit?

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You should start your own topic it gets too confusing.
Specifically; what do you mean won't fit?

Re: Mercruiser 120

dozerII said:

Thanks Steelespike. After looking at the prop I have, and going over what happened, I am thinking that the hub is slipping in this prop. Would this only happen when it gets to it highest load as in when trimming out?. The manual states WOT to be between 3900 and 4300 rpm. How much difference would going to a 14,1/4" diameter make from the 15, 1/4?
Thanks again for you help.
Glen

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If you had a spun hub trim wouldn't trigger it.The more power you put to it the more likely it will slip.You can double check by drawing a water proof line across the hub and test driving the boat. The messed up prop would not perform very well trying to trim out
or in hard acceleration. If a prop shop can fix the prop it will be more cost effective
and will give you a more accurate idea of your actual rpm. After tests if the prop appears to need a change keep it for a spare and we can probably help better with prop selection.
You seldom select a prop by diameter.Diameter is a function of the prop design and the only diameter consideration is actual fit.Some designs require that the anode under the antivent plate be changed.If the prop is listed for your application it will fit.

Re: Mercruiser 120

bifflefan said:

Have your redone at a prop shop for about 60 bucks then go from there.

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Thanks Bifflefan. I live in a remote part of Saskatchewan Canada and it would cost more for frieght and the rebuild that a new prop is worth.

Bifflefan, The Solas I ordered from Iboats 15 3/10 X21 seemed to not fit right. I called both Iboats and Solas about it, and the Solas tech guy seems to figure it will work fine. The rear brass thrust washer does not catch the splines on the prop shaft because the hub is .5 inches longer than the stock prop, and the prop ran smack in the middle of the steering trim anode. I ordered an offset trim anode and installed it this afternoon with the Solas prop and still had to file the anode back over half an inch for clearance for the Solas prop. The thing that bothers me is the lock tab for the prop nut locks into the brass rear thrust washer, if the thrust washer does not catch the splines the lock tab does nothing.

Steelespike. When I changed props today as mentioned above I inspected the hub in the old one and its is shinny in a couple of spots next to where the rubber meets the hub and I found a few shards of rubber. I will be taking the boat out again next weekend and will post the changes made by the Solas, it is the same size and pitch just a totaly different design, and I am a little worried about the fit.

Thanks again Glen

Re: Mercruiser 120

We took the boat out over the long weekend and put about 6 hours on it with the Solas 15&3/10 X 21 The boat seems to pull the prop okay but wot is only 3400 rpm and top speed is 32 mph. No more problems when trimming so the old prop must have had a spun hub. For cruising around I think this prop works pretty good. We tried pulling a two man tube and it did okay but laboured a bit in turns.
Glen