why is my Prop slip so high%!-gctid812631

[Guest]

My prop slip is ridiculously high, any ideas why?

Clean bottom, new props, my boat might as well be going through glue!

My boat is a 288 Bayliner 288 with a 4.2 D-Tronic 250hp.

I have been running the prop calculators and released that my prop slip is absolutely awful, my setup is

Bravo III 2.20 ratio leg, using a 22P prop, 3800RPM, a WOT speed of 22 knots

i have also tried a set of 25P props, 3600RPM a WOT speed of 24 knots, but it was a dog getting out the hole and felt like the engine was lugging through the rev range.

A cording to the calculators my slippage is really bad nearly 40% am i doing something wrong here???

I just ordered my self a new set of 22p 4x4 props from hill marine, hoping it should help with this slippage.

[bhawes]

I'm not familiar with your boat but I believe that you are asking a lot of a (relatively) small single engine to bring that boat up to speed. The wot of 3800 seems low for that engine, indicating that you should be running a lower, not higher pitched prop, but that would also bring your top end down even further.

[The_Other_Gary]

If you go to 4 blade you need to drop to 20 inch pitch, not the same as you have now.

[Dirtyface]

sounds like your right on the money...I had a 2004 285 w/300hp b3 22"pitch props and I was right at the same speed at that rpm. You have to remember that's a pretty heavy boat with marginal HP. The older versions used to have 330hp big blocks...FWIW. Cheers, Brad

[aluxury1]

The more under powered the boat is, the lower pitch prop. Gear it down! I have Bravo 2 18.5 x 19 prop 250 hp mercruiser 250hp 4.2L and the numbers are 3200 rpm 23 mph 6 GPH

[Guest]

Thanks for all the replies. spoke to Joe at Hill Marine and he decided 22p was best. The previous owner had changed the leg from a stock 2.00 ratio to 2.20 without changing props and its had the effect of raising the revs and sapping 10% speed, this was causing the boat to run against the limiter at WOT 3850rpm which was really heavy on the engine.

Looks like a 22p 4x4 should do the trick as they tend to reduce the RPM for the same given pitch of a 3x3.

I am hoping it should improve my slip numbers and in effect give me free speed for the same REVS. The boat feels currently really heavy in the stern, even when she is on plane it feels like the arse is ploughing due to the weight of the heavy diesel engine. Its impossible to run this boat without the trim tabs permanently full down.

With any luck the extra thust of the 4x4 should help lift the stern up higher and improve the efficiency of the Bravo III drive as its currently buried which is killing my performance.

That is at least what i think is happening

[desuch]

less pitch higher rpm

[aluxury1]

I have the same boat and power you do, just not the dual prop, I don't cruise with a lot of tab. I have been investigating going to a 24 x 12 tab also. Tabman recommended that size for our boats. I just want to stay on plane at the lowest speed possible. you will here the engine labor when you go to far in with the planes, and watch you speed with the adjustments. Previous boats I have owned, the tabs changed the RPM's, not so with the diesel, watch the speed and listen.

[mister_larry]

Running with the trim tabs "permanently all the way down" is NOT a good running attitude. Too little power, or tab, for the boat.

Take a look at the thread below about Adding Drop Fins to my Trim Tabs. There's quite a conversation about beefing up the trim system - as a place to start.

You can certainly buy new units from Tabman and put the old ones on Craigslist. Or you can drill some holes and bend some metal the next time you pull the boat. Whatever you do, the science says more wider trim tabs help, NOT longer. The deeper into the water they go the more resistance (drag) the create. A little self-defeating. Wider plates help life the stern.

Don't be surprised if a better operating trim system helps speed and efficiency more than experimenting with prop pitch and diameter.

Happy boating.

[Guest]

If i lower the trim tabs even going WOT she will go off plane, its all pointing to the props not delivering enough thrust for the weight to lift it out of the water properly. I am hoping these props might just do the trick touch wood!

[aluxury1]

I think you are over propped, what are the size of your trim tabs? They are more than likely 18x12 dropfin tabs. They work well on my boat so I don't think that is your problem. Call mercruiser or Hill Marine about the props and ratio. Less pitch will also mean less prop slip.

[mister_larry]

Something is a little confusing. You state that if you even touch the trim system at WOT she will go off plane. That implies that the boats attitude (relationship with the water) is at such a place that any more "bow down" increases the wetted surface. That sounds like either a speed issue (but you are at WOT) or a balance issue.

To get more speed you can try different props (like you are doing) or tweak the engine for some more performance. To adjust balance you can just look the boat over and see if there is any extra weight far forward that the previous owner added? Even though she is underpowered, one assumes that the hull design of the boat does allow for more speed once she gets on top. So why isn't this the case? Are you carrying serious anchor chain up forward?

Regardless of how quickly you plane, if there is no room for more speed while up there you are seriously limited in how you can use that boat. aluxury1 has indicated he can get a little more speed with an 18.5 x 19 prop set up. One wonders what load generates those numbers and how that stacks up to the usual load you carry. Guys with a family of 5 might have no choice but to get more engine power in order to achieve the attitude needed to plane safely. If you are forced off plane by merely touching the tabs I feel there's no margin for safety.

The major factors that go into performance of the semi-planing hull are dead rise and speed. If you want to plane, you need enough speed or a dead rise number that allows the boat to pop on top. Hopefully your prop experiments give you sufficient speed to get up (and stay) there. If not, the easiest way to change dead rise is with trim tab modification. It is possible to affect a change in dead rise by pressurizing the trim system to imitate a flatter dead rise. Pardon the reminder (and this will be the last one) but look at Capn Crunchie's thread on adding drop fins to his trim system.

That's another avenue you might need.

[aluxury1]

What speed are you at at WOT? What is the RPM? Also, reading back in the story, our boats are better balanced than big block boats. There are 4 less cylinders in the back and 2 more in front of the V8 block configuration. They also used a smaller fuel tank to allow clearance for the in-line 6 cylinder engine. My boat has no problems getting on plane.

[Guest]

Its confusing the crap out of me, the boat is definitely planing as I am traveling at nearly 24knots WOT, but like said previously i cant achieve that speed unless the trim tabs are buried. I have tried to do the same with a stripped out boat too with just minimal everything, no water, quarter tank of fuel, no gear. Its like the Bravo 3 with props is not providing sufficient lift to keep the boats stern out of the water.

The high slippage of my prop backs up this up of nearly 40%!

I have gone from 25p 3x3 down to 22p 4x4, i am hoping to see a significant transformation as the 4 blades should aid in helping reduce slip, provide more lift on the stern and lower my planing speed.

My only trouble is now trying to fit them, my yard lift is out of operation for 2 weeks and i want it done sooner. has aonyone had any experience of changing props in the water?

I was thinking of tilting the stern drive up so i dont drop anything undoing it and just use a snorkal.

[fishaman]

Changing propeller in the water is not a problem.

Did it many times, very easy (at least for Bravo 2).

Get a prop wrench, a screwdriver, and a small block of wood.

Raise the outdrive to trailer position, the props will be pointing up just under the water surface.

Drop your dinghy in the water, or if your dock is low, reposition the boat so that the stern is facing the dock.

Laying in the dinghy or on the dock, remove the props.

Don't drop any parts in the water.