Propellor Question

[Leadchucker]

Our 1996 Seaswirl has a Johnson 130 and originally came with a 14 1/2" x17" prop which the dealer said was the right wheel for the engine. After the engine got it's run in hours on it, if we went to WOT it would run right on up to 6200 rpm's . I asked the dealer at the time about it and he said it shouldn't ,but maybe I got a "strong" engine, so he agreed to send me a 14 1/2" by 19" wheel for cost and it brought the RPM's back to the upper range of the reccommended WOT range (5800 rpm's). I got almost 8 years out of that prop before I got tangled our own anchor chain running up on our rode last autumn,which buggerd up the prop. So I went a bought a new wheel of the same diameter and pitch, and now the engine will run up to almost 5900 at WOT. I don't think the hub is slipping, as I had this happen on another boat and know what that is like. If I put the guns to her, she don't sound like she's slipping and acts about the way she should. Anyhow,the question is, Could this prop be mis-pitched somehow? The props were all the standard OMC aluminium items. Would a 14 1/2" x 21" be too much for this engine under the conditions. Boat weight is still the same and all that. I know about the rule of thumb for 1" inch of pitch equals 200 engine rpms and such. I just didn't want to go a spend the bucks for a new wheel to experiment if there's something else in the mix I'm missing. Thanks.

[jawz12]

your guide of 200 rpm for inch of pitch-a little "optomistic" more like 150...any way....leave that prop on there-a 21"pitch will not turn up enough !
a marine engine has a reccomended rpm rating-the engine should operate in the upper end or slightly over the max rpm-now SLIGHTLY IS JUST THAT....SLIGHTLY AS IN 50 OR MABEY 100 RPM....the reason for this is simple,the engine must not be overloaded,by over loading an engine,what you've done is effect the durability of that engine by,raising exhaust temp-due to the fact the engine will not reach full rpm-on an outboard this can and will cause a "detonation " condition.it also effects the performance of the boat-too large a prop and the boat will be a slug out of the hole and take hrs to plane off,the speed will be down and the boat will suck fuel like a jet !!
water is 900 times more dense than air-with that , a boat is like a car allways running up hill,by allowing that engine to reach it's max rpm,you made that hill a little less steep....
don't be led by dock side "know it alls", too big a prop will damage the engine...
any other questions ?? feel free to drop me an email at jimmysmarineservice@comcast.net

[AhiSlayer]

Aloha!
Is the replacement prop cupped? if it is that could account for the rpm change. For a prop of like diameter and pitch expect a 200 rpm change by changing from cupped or not cupped
good luck
ahi

[74Formula233]

Just about all props are now cupped.....not many aren't, especially the "typical" applications such as outboards and most outdrives.

That said, even thought it is a factory prop, remember, it is 8 years later, and there may be a slight difference ..... different "batch LOL. Anyway, it could be as simple as a different amount of cup from your last one. The pitch is probably identical, but I have seen a great variance in cupping in props I have looked at, both factory and aftermarket.

I'd leave it alone. 100 rpms is not enough to make you want to change props.

[Leadchucker]

Thanks Guys!! She still makes 38 kts @ 5000 rpm's on flat water when I feel like going fast, I'll leave her alone.