heavy boat do you change the prop??

[emcm0025]

Hi everyone,
Just looking for a little input on the above

When you have a heavy boat and down on knots do you chnage up or down in prop pitch or do you save on the hassle and just live with it?

example

current prop 4 blade 3 adults on board 21knts at 3700 rpm
6 adults and large dog 3700 rpm 18-19knots, would that constitute for a prop change or just to leave be?.. Just asking this for general advice and views

thanks for any advice and info

Ewan

 

[Sultans_Kapitein]

[ QUOTE ]

6 adults and large dog 3700 rpm 18-19knots, would that constitute for a prop change or just to leave be?


[/ QUOTE ]

I think that at the end of the day if you often have 6 adults and a large dog onboard then maybe you should change . . . . . THE BOAT /forums/images/graemlins/laugh.gif


Mal

 

[lovezoo]

It does sound like you need a smaller pitch prop. If you have a Mercruiser engine there is a useful prop finder on their website which will work out what size prop you need:

www.mercmarinepropellers.com/

What engine and prop do you currently have?

 

[oldgit]

Assuming the boat to still have the same prop/hull/engine configuration that it left the factory with and the fact that the hull designer/engine builder/prop producer probably spent ages and a great wad of dosh on getting the best compromise between pitch and blade size,why fiddle.
Get it wrong and at best you will spend a shed load on a nice selection of props that will only end up on Ebay or at worst damage your engine.
Unless of course you know something that Rinker/Volvo/ Mercruiser etc have failed to spot over the last 50 years. /forums/images/graemlins/grin.gif

 

[Solitaire]

[ QUOTE ]
example

current prop 4 blade 3 adults on board 21knts at 3700 rpm
6 adults and large dog 3700 rpm 18-19knots, would that constitute for a prop change or just to leave be?.. Just asking this for general advice and views



[/ QUOTE ]

Simple, you have made the boat heavier! Don't mess with the prop. Changing pitch size to a smaller pitch will maybe get you out of the hole quicker, but you will lose top end. Putting a bigger pitch on will not get you out of the hole at all! Leave well alone.

 

[lovezoo]

[ QUOTE ]
Changing pitch size to a smaller pitch will maybe get you out of the hole quicker, but you will lose top end.

[/ QUOTE ]

Not true if the figures he quotes are for a petrol boat at WOT. If he is at WOT and only getting 3700rpm, the boat is overpropped. A smaller prop will give higher revs, more power and higher top speed. This also assumes the bottom of the boat is clean.

So a few things to check before swapping props, but in my experience on this size of boat, changing prop size and type can make a big difference and is worth looking into.

 

[emcm0025]

Sorry for any missunderstanding just realised I never upgraded my profile

The engine is 5 liter mercrusier on a bayliner 2655,
the wot rpm with that weight is 4100rpm witch is under mercs running limit.

Do i take off the 4blade and just swap it like for like for the same dia and pitch 3 blade?

sorry for the tedious post but feels like its been what feels like a long time getting the boat running so now im just trying to get it running spot on

 

[Falken]

When we owned our Spirit 3000 (now owned by Chris_D) it was fitted with Mercruiser 4.3 and Alpha One legs. I fitted a super set of Piranha plastic props for her, the blades just slotted into the hub. We kept a couple of spare blades and a complete set for when we were mucking about skiing. To change prop pitch took about 2 mins per leg. The other neat thing was the company offered an exchange service. Buy a set, try it and if the pitch was wrong take the blades back and they would swap them for you. Oh yes, they were dishwasher safe as well. /forums/images/graemlins/ooo.gif

Regards
Doug

 

[Solitaire]

[ QUOTE ]
A smaller prop will give higher revs, more power and higher top speed.

[/ QUOTE ]

If you say so! Yes, it will give higher revs but does not follow that top speed will be similarly incremental. It will dependnd a lot on the diameter of the prop as well.

It's really down to the fact that he has more weight on a small boat.

 

[PCUK]

Higher revs don't mean more power, just that the engine is running lighter due to less drive from the prop.

 

[capnjack]

First things to discuss before re-propping are....is the engine tuned correctly, is the hull clean and are you sure you are getting all the power from the engine? For example, lack of power can be attributable to bad manifolds, very often overlooked, only if you have a raw water colling...(opps bad spelling) "I mean cooling" system of course.

This all checked and sorted, WOT should be about 4000-4500rpm, so your boat is not far from this. Prop diameters tend to be set these days for sterndrives and outboards, as is cupping, rake and blade size. So the decision to change your prop will lie in the name and make and what it does, instead of trying to design one of your own i.e. changing the pitch and having the same diameter, cup and rake etc.

4 blades will be smoother than 3. They "usually" have the same top end but will pull better at mid range.

My advice, for what its worth is to have a look at the props available and then chat to a couple of specialists like Steel Developments.

 

[emcm0025]

It was Steel Developments who I went to about the 4 Blade which went over the ins and outs of each prop,

The manifolds thing poses an intresting point due to when was out some alarm has gone off twice on 2 different occasions now but couldnt trace it to where, all i do know is that engine heats more on the one side still working that out as sometimes its an even heat and others it's one side hot and one cold?? had new gaskets and a flush through, new impeller, new temp valve/unit still the same.

 

[peterb26]

There is no correct prop for any boat. This is simply because the environment the prop operates in differs on a daily basis due to weight and drag.

If you have the correct prop for (say) 3 people and full tanks and a clean bottom - then the engine will just peak at max allowed revs. Thats the correct prop for that loading.

However - that self-same prop will be too coarse for 6 adults, or even 4 adults and dirty bottom - and you wont achieve max revs.

If your boat wont achieve max revs with just 3 of you and a clean bum, then I'd suggest that its too coarse.

Its not clear from your post 1542442 if you mean it will only rev to 4100 with 3 people or with 6. Nor do you say if max revs is a lot more than 4100 so its difficult to judge.

 

[emcm0025]

Hi sorry for not making it clear its been a long weekend, boat wont achieve max rpm with 3 adults and a clean bum, and far from it with 6, the 4100 is full throttle.

Im now concered that the engine has some major problems as its burning up gas quicker than it can be filled and also that it cant get near mercrusiers running rpm range some times it does manage 4400 just and others 4200 and it tops out,

had new fuel filter 2 weeks ago new spark plugs, oil change is all thats left to do in the serviceable side of things that I can think of.
The running rpm for the engine is 4400 - 4800 according to the plastic top on the flame arrestor.

hope this clears things up

Ewan