Engine overpropped? Not sure on options/benefits of changing

I've just got my rebuilt coastworker out on the water for the first time since buying the boat and it was a real pleasure to use, but I'm not getting the engine revving out.

This is the boat - 18' console - similar to whaler I think:


Engine is a 1996 BF75A Honda, and as i understand it the WOT revs should be able to reach 6000rpm.
At full speed on the water I was hitting 34mph on the gps, 2 up, and very light load, at 5200rpm. Couldn't get it to rev higher.
This is with a 19" pitch 3 blade alloy prop, which I thought might be a bit much but only one I have currently.

This was on pretty much flat sea, pretty much perfect conditions for a speed run.


Now of course the obvious thing to do is try a 17" prop if I want the engine to rev higher, but what I'm not sure is if there's really any point or if 5200 is an acceptable WOT rpm?
If I change to a 17" prop, then I would guess the outcome will be I'll get the extra revs and be able to hit 6000RPM, but would then need to use the extra revs to maintain the same speed as before, so maybe still get 34mph, but at WOT and using more fuel I guess as a result?

I understand that if the boat has more people/load then the 17" prop would enable it to get on plane easier and reach WOT at any point, but it's not often I'll have more than 2 up, and I can't help but think that keeping the 19" would give better economy.

Appreciate any input and opinions from prop boffins as I'm not sure what's the best course of action.

Thanks!

There is another factor actually - which is that I'm trying to upgrade the engine to 90 from 75. I think with that extra oomph the 19" prop would then be ok and the engine would be able to rev out, and hit perhaps 37/38mph at 6000rpm. I need to get a 90 carb though as I only have 3 of them currently on my bench.

Re: Engine overpropped? Not sure on options/benefits of changing

The prop calculator indicates about 15% slip a little high but that could be a slight variance in the tach.
Calculator also indicates about 6000 with a 17" prop at about 35 mph.
Generally is accepted to run at or near max rpm especially if heavy loads are expected.
It does seem that a 90 would do well with a 19" prop.
I don't suppose its possible to modify the 75 carb to work in a 90 application.

Re: Engine overpropped? Not sure on options/benefits of changing

Didn't know there was a prop calculator tool on here, or is that elsewhere?

Anyway, mostly it'll be light loads - 2 people typically and some fishing kit plus basic spares/tools/fuel/safety stuff.

It might be possible to modify a 75 carb - not sure. I believe they are basically the same carb from comparing them except that the 90 one (40AA) has different jetting, and a much wider throat than the 75 carb (35AA).

Might be possible for a decent machine shop to 'bore out' the extra aluminium choking down the 75 model, but I'm not sure how easy that would be, and I'd guess that would cost more than the cost of a used one.

I've already bought one '90' set which I had to return because they were 35AA numbered when they arrived (off a 75 not a 90 as claimed), and when I finally got a 2nd set, it turned out to be 3 from a 90 and 1 from a 75!! The guy claimed the engine was fine and he never even knew it had a 75 one in there, but I'm not going to run it like that. Would place an uneven load on the crank, without even considering mixtures etc.

So at the moment I've got the engine on the boat running 75 carbs (is actually a 75 engine), a second complete running BF75 engine for spares, and a set of 3x 90 carbs with 1x75. I've got a bit of a carb surplus here but need one more 90 one from somewhere.
Would happily trade a part from the spare engine to get a 90 carb or buy if available for sensible money.

Re: Engine overpropped? Not sure on options/benefits of changing

personaly i prefer a little over prop but i do not stay at max rpm while on plane all the time. in small chop i dont like the idea of the engine reving higher than the max rpm rating when the prop starts to break water. trim/motor height might get you a few more rpm as 15% slip is a little high but that might be the condition of the prop. if you have a boat yard that does used parts near by you might be able to rent/borrow a few used props to test with. not all props are the same so a 19 ss might get you 2-300 rpm with less slip. until you change the carbs i would forget about the prop for now

Re: Engine overpropped? Not sure on options/benefits of changing

Fair enough thanks. Will a stainless one gain additional RPM for the same pitch then or lose RPM? I don't know a lot about them but if they're stiffer it'd result in a lower (under load conditions) pitch than an alloy prop presumably and thus gain the RPM increase?

Might be thinking about it the wrong way but as the blade is pushing against the water and stainless flexes less, I'd expect that means closer to the stated pitch, whereas an ally one effectively gets more pitch when it flexes in theory?

Not likely I can get a prop to borrow I don't think as it's a pretty small area here and options like that very limited. I can try asking on local classifieds though in case anyone has one to buy cheap.

I think you're right about the carbs. Got to find one first though.
Looked on the classified section here and can't see anything. Is there no 'wanted' ads on this forum or am I being blind?!

Re: Engine overpropped? Not sure on options/benefits of changing

IMHO I think that SS blades are usually thinner than AL and that has a lot to do with why they are more efficient. When you get more HP the 19 will likely be OK. However in the interum if your not running in the band of reccomended WOT RPM arn't you lugging the engine at all speeds? I would get a 17 to get you into the manufacturer rec WOT range which I think is 5500-6000 so I dind't lug my engine at any speed.

Re: Engine overpropped? Not sure on options/benefits of changing

Thanks for the info.

Just checked the engine specs and it shows the stated 75hp at 5000-6000rpm WOT, so technically it's in spec but the lower end of the range. Max torque on the other hand is at 4000rpm.

Might have the offer of a cheap stainless prop but I don't yet know the spec.

Re: Engine overpropped? Not sure on options/benefits of changing

Very basically as prop speed goes down slip goes up.So the 5200 may account for some of the slip.

Re: Engine overpropped? Not sure on options/benefits of changing

Georgesalmon said:

IMHO I think that SS blades are usually thinner than AL and that has a lot to do with why they are more efficient. When you get more HP the 19 will likely be OK. However in the interum if your not running in the band of reccomended WOT RPM arn't you lugging the engine at all speeds? I would get a 17 to get you into the manufacturer rec WOT range which I think is 5500-6000 so I dind't lug my engine at any speed.

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engine wont lug unless its being asked to work hard so its fine at lower rpms it just not giving you the max performance that its capable off. with 19' bass boats running 200hp plus it would be fun to watch if they could only run at tick over or wot. once on plane most people back the engine down to the 4k range as theres no need to burn a major amount of gas for a few more mph

Re: Engine overpropped? Not sure on options/benefits of changing

Just a little update to keep this thread potentially useful to others, that I finally managed to upgrade the engine from BF75 to BF90 with the fitment of BF90 carbs.

After a bit of initial hassle thinking I had a 'problem cylinder' because I hadn't syncronised them, she runs great as a 90.

On the water, this 15bhp increase (same 19" alloy prop) translates to much better holeshot, and 5mph top end increase. More significantly than the speed though is that the engine now revs out to 6000. If it's tilted up then that increases to 6200rpm but that's too much so I reigned it in.
It's possible that I could actually go to 21" pitch, or benefit from a stainless 19" prop, but at this stage I'm really not bothered about messing with it further unless I get lent props to try out, or get them for a steal. Overall, I'm very happy with the performance to be hitting 40mph (2 up).