Props and ventilation/cavitation

[Wide Open II]

Have been trying to get a definite answer on this for a while now. I'm currently running a 25 Mako with twin Opti 150's swinging 17p 4 blade Merc Offshore props (came with it). At WOT, I'm running about 40kts at 5600rpm. Makes good turns and doesn't break in tight turns. I'm one hole from being completely on the transom with the mounts.<br /><br />Here's the rub. From dead stop, I advance the throttles for my hole shot (not trying to set any records either) and as she comes up to plane, usually the starboard engine will "break" free and rev up considerably past the port engine. Trim tabs are up, nothing dragging, weight balanced, etc. Then I either have to lower the starboard throttle to let the port catch up, or wait about 8 seconds for the starboard engine to "catch". If it were an automatic transmission car, I would say it's getting the next gear. <br /><br />The hole shot is not particularly important to me, but in a following sloppy sea, trying to maintain headway and control becomes difficult, when the props are ventilating. Suggestions?<br /><br />It's hard (no, impossible) to find blue props for these engines on that boat where I'm located to be able to change out and see if it's a prop issue. Think lowering the engines a hole would help. Again, I don't have a hoist, so I'm reluctant to spend the money to have a shop lower them, then raise them again. <br /><br />Thanks<br /><br />Wide Open II

 

[Forktail]

Re: Props and ventilation/cavitation

I'd lower them. Your speed looks excellent for that setup, and I doubt you'll lose more than a mph or two. Otherwise look at a prop with more cupping and less pitch.

 

[ob]

Re: Props and ventilation/cavitation

I'd go ahead and just lower it the one hole you have left and water check.I'm assuming you are trimmed in all the way when this occurs?

 

[dick]

Re: Props and ventilation/cavitation

One more question is the port engine counter rotating ?

 

[Wide Open II]

Re: Props and ventilation/cavitation

Yes, port engine is the counter rotating engine. Guess I'll go ahead and lower them one notch.<br /><br />Wide Open II

 

[12Footer]

Re: Props and ventilation/cavitation

that sure sounds like a spun out hub. If the hole lowering doesn't cure it, I can allmost garauntee the hub is slipping. Also, if you have recently mounted anything on the hull exterior, that could cause air bubbles to ruin the water. Any dents in the hull could cause it. But the hub is the weakest link here, and therfore, your primary suspect. If the engines were performing normally before,and all of a sudden they aint, you can rule-out trim issues pretty much.<br />The fact that it 'catches' would indicate that the hub is still intact,and may "look fine". But under heavy loading, it is designed to slip. Too much hole- shooting, grass-grazing, or running into manatees can cause it to slip, and keep on slipping.<br />Sometimes, you can see the hub is deformed. It (being a rubber donut in a metal housing) may be visably out in some spots --- and in , in others.<br />So I would (if it were my rig, and wish it was,sounds sweet)--I'd remove both props, and send them to the nearest prop shop for re-hubbing. This should cost under 50buks per, and more like 40 buks per.<br /><br />.

 

[Hooty]

Re: Props and ventilation/cavitation

Spun Hub.<br />If you think the hub is spinning, put an alignment mark on the prop nut, rubber bushing and prop hub and see if they move.<br /><br />c/6<br /><br />Hooty

 

[KIWI MERCMAN]

Re: Props and ventilation/cavitation

12 footer<br />I've never seen a prop hub that slips sometimes and not others, with 150hp swinging it it's either good or bad. I'm not saying that it might not be possible, just that I've never seen it.<br /><br />Go ahead a drop them, you can do it with out a hoist, by walking them down your transom one hole at a time. Take out 3 bolts and leave 1 top in tight. Slowly loosen the last 1 and the engine will lay over, and will reach the other top hole so you can replace that bolt.Once done remove last bolt and the engine will sit down to align the rest.<br /><br />Kiwi

 

[MGuckin]

Re: Props and ventilation/cavitation

Swap the props from side to side. If no change, drop the motor.

 

[ob]

Re: Props and ventilation/cavitation

Can't do that.They're counter rotating.

 

[12Footer]

Re: Props and ventilation/cavitation

KM, Not that rare to be intermittant.<br />They can act like a clutch in a car. If you spin the hub out, sometimes you can make it back to port by nursing it up onto plane, by slowly increasing the throttles, until it begins to slip, then back-off just a hair to 're-catch', and then slowly increase some more.<br /><br />I once hit a coconut,spun-out the hub, and continued fishing all day by nursing the binacle.<br />Granted, it was only a 50HP on a 17ft boat,and I couldd only reach about 20MPH, but that only matters with horseshoes and hand grenades.<br /><br />At the price of labor to lower the engines, it would probably be cheaper to pull and ,at minimum, inspect the hubs/props/spines.<br />Besides, if it used to work ok at the present hieght, it stands to reason something else 'happened' , like a hub failure, or hull dent , happy growing hull critter ,or sompin.<br />All it takes is one well-situated barnicle, and it'll cavitate like mad. That was hardly worth mentioning tho, as he checked that allready.

 

[MGuckin]

Re: Props and ventilation/cavitation

OB.....thanks didn't see that.<br />

 

[Hawkeye1]

Re: Props and ventilation/cavitation

I hate to tell you this but I tried a bran new merc offshore 4 prop on my boat and it was the most cavitating prop I have ever ran when comming up on plane. I had way better grip and performace with my stock suzuki 3 blade and a merc mirage. I think the rev 4 are the cream of the crop for merc 4 blades but never tried them.

 

[Wide Open II]

Re: Props and ventilation/cavitation

I have checked the boat from gunwale to gunwale for any sign of something that would spoil the water flow. Aside from the designed-in-to-the-hull saltwater inlets and bait tank inlets, it's darn slick. It has Bennett trim tabs in pockets, they are polished and no dents or dings. I have two transducers mounted on the transom. I moved them a little further apart and lower on the transom this past summer...BUT the problem existed prior to that, and with the move there appears to be no change positively or negatively. As a matter of fact, this has pretty much been the case since I had the boat delivered. The dealer switched out props a couple of times when I first purchased it and these were the best performing out of the ones we tried. Initially it has 21p 4 blades on it.<br /><br />I really don't care that much about speed. I want to be able to have control when things turn sloppy and need to come in on the back of a wave. Constantly nursing throttles because of cavitation is a pain, plus when you really need that little boost of power, and you have a delayed re-action due to "slippage", it's even more distressing. <br /><br />I don't think the props are slipping on the splines, but I'll check. Just to verify, do I leave the engine in gear, hold the prop nut, and try to turn the prop?<br /><br />Thanks to everyone. Very helpful

 

[ob]

Re: Props and ventilation/cavitation

To remove prop lower engine ,leave in neutral,and wood block the prop blade.You may be able to perform a close visual on the props installed and see if one is spun.If visual doesn't reveal anything put two aligned scribe marks ,one on hub and the other on prop barrel and load test.Verify whether marks stay aligned.<br />Where exactly is this engines anti-ventilation plate located in relation to the bottom of transom when trimmed level with hull?

 

[Hawkeye1]

Re: Props and ventilation/cavitation

Again I would try a different prop. I thought the offshore 4's performed very poorly in the cavitation area. I know others that had the same problems with those props and went to the rev 4 or thr Power tech 4 and got great results.

 

[12Footer]

Re: Props and ventilation/cavitation

Hooty has the best way to diagnose a hub problem.<br />Put a mark on the rubber of the hub,across the metal of the prop, with fingernail polish or simular paint, a perminent marker, or scribe, let it dry, then take it for a cruise. When you return, inspect the marks. If they have moved at all, the rubber hub is slipping, and should be rebuilt.

 

[GetReel]

Re: Props and ventilation/cavitation

So what was the problem ?

 

[Texasmark]

Re: Props and ventilation/cavitation

Just my 2c. Merc says the best bite comes with a single blade. Well you can't do that cause vibration would be prohibitive. So the next most efficient prop, according to them, is the 2 blade. Makes sense as the fewer the blades, the fewer the instances of turbulence.<br /><br />Now for these big dog bassers, they only run ONE engine and all the thrust has to come by that one prop, and that one prop is what's moving ALL the water. Big difference from your situation where one can carry the load and allow the other to loaf.....hence cavitate. Plus they (BBoats) get up and away which makes for lots of water flow across the prop which keeps it clear of bubbles.<br /><br />Why don't you try this B4 you do anything. Go out and run with one engine kicked up out of the water. Go thru your hole shot and get into some following seas. If she works good swap engines. If it works good too, I'm going to step out and say you need to drop down to a pair of 3 blade ss with high rake and good cupping.<br /><br />One other thought. If you have ported props the port holes could be working against you. I got this from what you said about shifting gears when the prop catches. Having run a ported prop on a basser, I know the feeling when the bubbles sweep and the blades (3 in my case) bite clean water. <br /><br />With the thrust of 2 engines coming out of the hole, you may not want port holes in your props to improve your hole and in following seas, you very well could be forcing exhaust out the ports which contribute to the cavitation.<br /><br />Also I don't think dropping your engine down to the last notch is the answer. Why? Because you said turns are ok. Well in turns, when you say it's ok, you are probably running water fast enough across the prop to keep the holes covered up and apparently hull disturbances are not bothering you, cause that's where you'll get them....so height is not your problem.<br /><br />I never ran 2 engines, but I can think through problems and hopefully I have said something here to help you.<br /><br />Mark