O/B height on single engine cat?

N3UP

Petty Officer 2nd Class
Joined
Aug 14, 2010
Messages
125
I'm not sure if this is the right area to post this but since it involves the prop, here goes.

I have a Leisure Cat 26 with a Mariner 225.

I have had intermittent problems with the prop breaking loose in light chop.
When it does, if I back off the throttle, it will hook up and go.
Trim seems to make no difference.
Smooth water in the river is no problem, but whe I get out in the bay with 1or 2 footers, it will sometimes break loose.

The prop is a 15 1/2 X 17P SS Mirage plus (3 blade)

On a twin engine cat, or monohull, I have read that the anti-vent plate should be just out of the water on plane.
With my hull, there is no way you could achieve that due to the wake from the sponsons coming out of the tunnel. To make matters even more interesting, my hull has a "pod" just forward of the motor, making an even more interesting wake situation.

I also suspect, I may be slightly over propped.
I get 5000 rpms at wot and about 36mph or so. WOT for my motor should be 5000-5500 I believe.

I'm not looking to gain any speed, just trying to eliminate the break loose problem.

Should I have a 4 blade prop in this setup?

The boat is 2 hrs away, but I will try to take some pictures of the current setup and post after the weekend.
__________________
 

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,544
Re: O/B height on single engine cat?

Obviously you are getting air into the prop blade pickup area. You need a solid stream of water across the blade to hold on. Blades like "Cleavers" for example, are built to run partially submerged at very high speeds.

The sponson spray in the tunnel is definitely part of it as it contains bubbles.

The hump preceeding the engine mounting is possibly there to reduce the spray and incoming waves to more of a solid mass of water like what would be in front of an engine in a monohull.

The Mirage is a high performance prop for recreational boaters and should do you as good as anything out there I would think.

Sounds like you could stand lowering your pitch a couple of inches using an estimated 200 rpm per inch as a guideline to get you near top end on rpm's. This will certainly help your engine as they run better at the upper end. Additionally, less pitch my help you in solving your problem. Probably will, but I wouldn't get your hopes up....just be glad if it does. To keep everything else in proper perspective, same Mirage + SS, just drop the pitch 2". I don't know if a prop shop can do that for you but you might ask. This should have a big impact on improving your hole shot from where it currently is, but even though rpm's will go up, mph may (probably will). Like what is the difference between the ratio 2/17 vs the ratio 200/5000.....12% drop for the forward distance per rev, while the rpm's only increase 4%. May cost you a couple of mph......buttttt what matters to you?

Last, if your prop is ported (one hole beneath and near the front of each blade) they may be the source of your problem. If so, you might take the prop off, get some epoxy and fill them in. If it doesn't work you can take a hammer and punch and knock the plugs back out. You will loose some hole shot however.....take longer to get on plane.

Best I can do.

Good Luck

Mark
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: O/B height on single engine cat?

Have you tried moving weight around.A person moved from the bow to the stern maybe even as little as 100 lbs of people or gear.Moving 100lbs from bow to stern is almost like adding 200 lbs.at the stern.It may even be possible moving weight to port or starboard might help.Usually a 4 blade will resist venting better than a 3 But would be a pricey test unless you can find a dealer that will let you test with the intent to buy the successful prop.Usually the 4 blade will plane quicker and stay on plane with less throttle also may handle better at the dock.
If the tach is accurate dropping to a 15 4 blade could add about 300-400 rpm.
Any time you change design and or manufacturer actual results can be more unpredictable.
Of course you could just lower the motor a hole or 2 it may not take much to solve it.
 

N3UP

Petty Officer 2nd Class
Joined
Aug 14, 2010
Messages
125
Re: O/B height on single engine cat?

The Mirage + that I have is not ported.

I have no issues with holeshot or time to plane, it's a big ol' heavy deckboat, not a go fast drag boat.

I think I may try to drop the motor down one hole since all that costs is a bit of time.

I am not looking to gain any speed, but I'd rather not lose any of what I already have.....
 

reddogg

Petty Officer 1st Class
Joined
Jun 3, 2011
Messages
379
Re: O/B height on single engine cat?

x2 on lowering the motor a bit. I'd take it an inch at a time and test the results. Too low and you will loose some rpm and speed.

Red
 
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