Your thoughts on easy revving engine..

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wipe_out

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Hi All..

Took "SEAFARI" out for the first time this evening.. Water off Brighton was amazing!!

Anyway, she has just had a new 5.7 V8 engine from Repower-Marine fitted paired with the existing Volvo SX out drive.. It's the complete engine with the Holley 4 barrel carb and I replaced the exhaust manifolds and risers as well so from where the fuel enters the fuel line to where is exits the exhaust is all new..

The engine needs some tuning because cold starting is a cow but between the engineer that fitted it and myself I am sure I will have it running brilliantly this weekend..

Here is the issue.. The engine seems to rev very freely.. I am new to this type of boat and haven't been on a boat in about 15 years so maybe its just me..

I was going along slowly and decided to get her up on a plane so I rolled on the throttle and she very easily revved to 4500 rpm BEFORE I REACHED FULL THROTTLE.. I backed off to about 3500 rpm as she was on a plane it was fine..

Looking at the manual that came with the engine this evening it says the max rpm is 4300-4600 rpm..

Now I remember on boats with outboards that I spent half my life on as a teenager when we wanted to get up and go we would just roll on the throttle to full and the boat would come up on a plane and fly along but I don't remember the feeling that it was over revving like I did this evening..

She is a 25'6" cruiser, I think in the region of 5500lbs.. I had three people on board, full water and probably about half a tank of fuel..

Now I believe the original engine the boat came with was a Volvo Penta 5.7GS.. From what I have been able to find these engines were about 270hp.. This new engine is 315hp according to the manual.. So 45hp more than the original.. Assuming the previous owners didn't change the props I am guessing that the prop is setup for 270hp and not 315hp and that could be the cause of the easy revving.. Does that seem logical?

I should have written it down but I think, from looking at the prop when she was out the water, the prop is a 15x17.. Does this sound about right for a boat this size?

Of course, perhaps inboards and out boards work differently and I am just not used to it so need to just watch the rev guage and back off the throttle before she over revs..

Your thoughts welcomed.. :)
 
Defo sounds like the prop is wrong. Some proper experts will be along shortly no doubt and will enter your intimate details into their computamabob and tell you what you need :D
 
It's really simple to sort out a prop, well the pitch at least! Run the boat in ideal conditions flat out and make a note of the RPM and the speed - if the engine hits the rev limiter (assuming you have one) you'll still need the information. If the engine is looking like going beyond the manufacturers recommended maximum RPM take the speed at the maximum allowed RPM. There are a number of free prop calculators on the internet and you also need to know the ratio of your drive (you can get this from the manufacturers website or from the drive itself), this will then tell you what pitch prop you need for what speed.

If it's only revving to the maximum recommended revs at Wide Open Throttle (WOT) but spinning the prop under hard acceleration if you have holes in the hub of the prop you can also block these up, they are there to help engines get to their operating power band before gripping.
 
We had a 25ft boat with less power (Merc 5.0l mpi giving 260hp) and a dry weight of 6350lb. we fitted a 16x16 prop which was a good match. We had a 15x17 as a backup with similar experience to yourself other than it struggled on the the plane when fully loaded with that prop fitted.

Give this tool a go.. It's Mercury but accepts your Volvo engine and drive as a feed in to the tool. It should give you a good idea whether the current prop is still ok other whether you can go for higher pitch / diameter given that you now have more power to play with

http://www.mercurymarine.com/propellers/prop-selector/#
 
Took her out again today.. Fully loaded with 5 adults and "stuff".. Sea was a bit rougher too..

Only way I can explain it is it felt like there wasn't enough "traction".. Tried to get going after coming out of the marina, rolled on the throttle and the revs climbed easily and steadily to about 4300 but didn't feel like it was "pushing" the boat onto a plane.. Just sort of spinning away merrily.. Might be cavitation but it didn't feel like it was pushing and then stopped pushing.. It just increased the revs steadily as I added throttle..

I will have to speak to the guys who just put the engine in and see if they have any props we can try.. Seems like the next step from the 15x17 is the 14.5x19..
 
wipe_out said:
Took her out again today.. Fully loaded with 5 adults and "stuff".. Sea was a bit rougher too..

Only way I can explain it is it felt like there wasn't enough "traction".. Tried to get going after coming out of the marina, rolled on the throttle and the revs climbed easily and steadily to about 4300 but didn't feel like it was "pushing" the boat onto a plane.. Just sort of spinning away merrily.. Might be cavitation but it didn't feel like it was pushing and then stopped pushing.. It just increased the revs steadily as I added throttle..

I will have to speak to the guys who just put the engine in and see if they have any props we can try.. Seems like the next step from the 15x17 is the 14.5x19..
Click to expand...

also check the prop's not slipping on a plastic hub if that's the setup you have
 
Just been out for a short ride again today and have some numbers that may be of use.. According to my phone GPS (speedo doesn't work at all) we were doing 18.8 knots at about 3500rpm (assuming tacho is accurate since nothing else seems to be).. For a 25ft cruiser with a 315hp 5.7l petrol V8 I would expect a top speed somewhere around 35 knots.. Even if we call it 30 knots surely it unlikely to pickup another ~12 knots in the remaining 1000rpm of range before it reaches max RPM right?
 
wipe_out said:
Just been out for a short ride again today and have some numbers that may be of use.. According to my phone GPS (speedo doesn't work at all) we were doing 18.8 knots at about 3500rpm (assuming tacho is accurate since nothing else seems to be).. For a 25ft cruiser with a 315hp 5.7l petrol V8 I would expect a top speed somewhere around 35 knots.. Even if we call it 30 knots surely it unlikely to pickup another ~12 knots in the remaining 1000rpm of range before it reaches max RPM right?
Click to expand...

To make a useful suggestion I would still need to know what it's doing at wide open throttle sustained until the speed evens out, you could try a larger pitch prop (keep to the same diameter if possible as a smaller diameter will bring the revs up again), but you would be shooting in the dark. For what its worth I think you're right about needing a larger prop but until you get all of the numbers required it's impossible to know what will work. Another option is to try and find someone with the same or very similar boat and engine combination to compare your setup with - What boat do you have?
 
Cookee said:
To make a useful suggestion I would still need to know what it's doing at wide open throttle sustained until the speed evens out, you could try a larger pitch prop (keep to the same diameter if possible as a smaller diameter will bring the revs up again), but you would be shooting in the dark. For what its worth I think you're right about needing a larger prop but until you get all of the numbers required it's impossible to know what will work. Another option is to try and find someone with the same or very similar boat and engine combination to compare your setup with - What boat do you have?
Click to expand...

I am worried about even attempting a WOT run because I am pretty sure it will rev way over the specified maximum..

Boat is a Larson 254 Cabrio that has just been re-powered with a 315hp (according to the book) 5.7l V8 linked to a SX drive.. I think the ratio on the drive is 1.43:1 but would need to double check..
 
wipe_out said:
I am worried about even attempting a WOT run because I am pretty sure it will rev way over the specified maximum..

Boat is a Larson 254 Cabrio that has just been re-powered with a 315hp (according to the book) 5.7l V8 linked to a SX drive.. I think the ratio on the drive is 1.43:1 but would need to double check..
Click to expand...

I can't see how any harm would come to the engine if you took it up to the max RPM - it is designed to do that anyway. If we have those numbers plus the ratio and prop pitch we can work out how much we need to bring the speed up and the RPM down.
 
Cookee said:
I can't see how any harm would come to the engine if you took it up to the max RPM - it is designed to do that anyway. If we have those numbers plus the ratio and prop pitch we can work out how much we need to bring the speed up and the RPM down.
Click to expand...

I'm worried it will rev to 5000+ which is what it feels like it wants to do..

Ok, if I get out ofer the weekend and the water is relatively flat I will give it a try and post back the results..
 
You are on the right way!

First check to see if full throttle on the remote actually makes full throttle on the carb.

Then load the boat to the equal of your normal load of persons, fuel and gear.

Trim the sterndrive all the way down (also referred to as 'in').

When in the clear give her WOT and see what rpm you get.

Changing pitch on the prop will reduce rpm at WOT by 200 for every inch and as recommendation is a range with 4600 rpm max, you can calculate the needed pitch.
If scared by the rpm you'll need to begin with identifying the current pitch and get/borrow one with higher pitch for testing. Pitch normally comes in increments of 2".

Note: Once on plane you should trim the drive out until the bow raises a bit, but not so far the engine starts revving without adding speed (prop drawing air). As less hull will be in the water this attitude will allow rpm to increase further so another increase in pitch may be needed.
 
Probably of no use to you, but our vp43 with sx1.79 ratio drive has 15x17 prop 190bhp tadge over 30 knots approx 4600rpm.
It did have a rubber cush drive inside the prop itself, this same with the new prop.
Cant remember the supplier but was recommended by this forum, in london i think, they also repair props.


Lynall
 
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