Engine height

[timdan94]

I have a 2010 crestliner superhawk 1700 with a merc 115 4 stroke. I'm having issues with getting the engine to run full rpms with a reasonable prop. I would like some opinions on the set up of my motor. here are a few pictures

Does my engine sit lower than it should?
Thanks

 

[fishndirk]

Re: Engine height

Looks like its close, maybe a tad bit low (less than an inch) if it is. What do you mean by "reasonable prop"? What's your prop size and RPM's at WOT? Does it struggle to get out of the hole? Does messing with your trim have any sort of effect on your RPMs at WOT?

 

[steelespike]

Re: Engine height

If your not experiencing any ventilation(cavitation) issues raise it one hole at a time till you find a point just before venting becomes a problem.
You may not gain a lot but at least you will know the least amount of motor is in the water.
Tell us about your problem and give us your wot rpm and speed with a light load.

 

[timdan94]

Re: Engine height

If your not experiencing any ventilation(cavitation) issues raise it one hole at a time till you find a point just before venting becomes a problem.
You may not gain a lot but at least you will know the least amount of motor is in the water.
Tell us about your problem and give us your wot rpm and speed with a light load.

13X17 black max 39-40mph @ 5700 rpm
The boat has very slow plane time with 2 people and full live wells and maxes out at about 5200 rpm. With just me it is still slow to plane and only hits 5700 rpms.The black max looses bite really easy when trimming out.

13X15 4 blade solas rubex 38mph @ 6800 rpm
The solas bites really well but allows the engine to over rev.
It will even hit 6800 with 2 people and full live wells

I bought a 13x17 4 blade solas rubex and could only hit 38mph at 5400 with just me in the boat with 3/4 tank of fuel.

 

[steelespike]

Re: Engine height

Not sure about the 1100 rpm difference of course a brand change is part of it and a design change,of course a 2" pitch change.
I would say your 13 X 17 numbers are just about as expected but should include quicker hole shot, stay on plane better at reduced throttle,may be smoother and more responsive around the dock.
Typically it is suggested too drop one inch in pitch when going to a 4 blade.This would make up for the additional blades drag
helping to maintain rpm.

 

[timdan94]

Re: Engine height

Not sure about the 1100 rpm difference of course a brand change is part of it and a design change,of course a 2" pitch change.
I would say your 13 X 17 numbers are just about as expected but should include quicker hole shot, stay on plane better at reduced throttle,may be smoother and more responsive around the dock.
Typically it is suggested too drop one inch in pitch when going to a 4 blade.This would make up for the additional blades drag
helping to maintain rpm.

13X17 black max 39-40mph @ 5700 rpm
The boat has very slow plane time with 2 people and full live wells and maxes out at about 5200 rpm. With just me it is still slow to plane and only hits 5700 rpms.The black max looses bite really easy when trimming out.

13X15 4 blade solas rubex 38mph @ 6800 rpm
The solas bites really well but allows the engine to over rev.
It will even hit 6800 with 2 people and full live wells

I bought a 13x17 4 blade solas rubex and could only hit 38mph at 5400 with just me in the boat with 3/4 tank of fuel.

The 4 blade 13x15 and the 13x17 are same prop manufacturer and prop series. I'm confused on why I lost 1400 rpm with only a 2" pitch change

 

[smitty477]

Re: Engine height

"The 4 blade 13x15 and the 13x17 are same prop manufacturer and prop series. I'm confused on why I lost 1400 rpm with only a 2" pitch change"

That does not make sense at all , but neither does 6,800 with a rev limiter. And neither does 38 mph with a 15" pitch at 6,800.
Something is really amisss - tach, prop markings, throttle connections?

 

[steelespike]

Re: Engine height

"The 4 blade 13x15 and the 13x17 are same prop manufacturer and prop series. I'm confused on why I lost 1400 rpm with only a 2" pitch change"

That does not make sense at all , but neither does 6,800 with a rev limiter. And neither does 38 mph with a 15" pitch at 6,800.
Something is really amisss - tach, prop markings, throttle connections?

Smitty makes an excellent point. You have had 3 wide ranging rpms yet esentually the same speed.
I'm wondering about the tach. Maybe retest the 15 again.
Some quick prop calculations black max 17 can't go 40 at 5700 rte quires negative slip,impossible.
solas 4 blade 15" 38 @ 6800 is 8% slip a good number.Except where is the rev limiter?
Solas 4 blade 17" can't go 38 @ 5400. Again negative slip.
it appears either speedo or tach one or both are wrong at some point.

 

[louiefl]

Re: Engine height

Aside from the confusing rpm / mph issues raised above, what are you trying to achieve? If you are trying to go faster than 38-40 mph, I don't think the problem is the prop, you need a bigger engine. If it is all about hole shot, does the 13x17 4 blade gets you out of the hole quick enough? 38 mph at 5400 rpm is about what you should expect for that engine and boat size.

 

[timdan94]

Re: Engine height

Aside from the confusing rpm / mph issues raised above, what are you trying to achieve? If you are trying to go faster than 38-40 mph, I don't think the problem is the prop, you need a bigger engine. If it is all about hole shot, does the 13x17 4 blade gets you out of the hole quick enough? 38 mph at 5400 rpm is about what you should expect for that engine and boat size.

The engine max rpms is supposed to be between 6000-6400 on that engine so I'm trying to get it into that range.

Smitty makes an excellent point. You have had 3 wide ranging rpms yet esentually the same speed.
I'm wondering about the tach. Maybe retest the 15 again.
Some quick prop calculations black max 17 can't go 40 at 5700 rte quires negative slip,impossible.
solas 4 blade 15" 38 @ 6800 is 8% slip a good number.Except where is the rev limiter?
Solas 4 blade 17" can't go 38 @ 5400. Again negative slip.
it appears either speedo or tach one or both are wrong at some point.

I'm using gps to get the speeds. I'll have to check the tach out. Thanks guys

 

[Jlawsen]

Re: Engine height

I see the first problem and it is with the motor height. Raise is up so the bottom of the cav plate (the first one above the prop) is 1" above the bottom of the keel. That motor has 20 degrees of trim so it will trim way up quick. With that boat, start your hole shot trimmed all the way down and when it's up on plane just tap the trim switch until your speed is maxed out then tap it back down once so it will hold in the turns.

The original 13 x 17 is probably your best prop. You're dealing with a 4 stroke now so throw out that old theory about needing to be running at max RPMS. 40 mph is about tops for that hull length and deadrise so I wouldn't be disappointed at all.

There are definitely some inconsistencys in your tach readings and that isn't right. You have SmartCraft on that motor and you could invest in the Eco gauge with troll control. I did and I love it. It basically replaces all your gauges and also interfaces with NEMA 2000 networks. I have mine setup so all my gauges are on my Lowrance HDS-8 and I can monitor everything even fuel. I know that would solve your issues with the incosistencies.

 

[Texasmark]

Re: Engine height

I agree that then engine needs to be raised and you have 3 open slots in your transom bracket to do that. The other thing that raising the engine will do is reduce the tendency for it to dig your transom in. Just look at your trim position in the second picture for example. You have a shovel back there headed for the deep six. The more you trim out the more it digs, to the point that it could easily be part of your overall problem with top speed and diversity in your readings.

I realize you have a steep deadrise, but still, once you get that antivent plate above the water coming out from under the boat, the boat will set the attitude and the prop will do more of a propelling job and not so much an attitude determining factor. You will let the hull perform the way it was designed.

Expect your rpms to come up when you do this (raise the engine) and you may have to run with reduced trim to keep from blowing out, but as stated, that may allow for increased top speed for a given trim level as compared to your current position. I'd put on a prop, take data, analyze it and put on another. Obviously if you have a higher pitched prop running you at/near redline, you don't want to put on a lower pitched prop.

Other thing is that this is cupping is important. The better the cupping, the less your chances of blowing out at a given trim level.

I have personally experienced what I have said in here about engine heigh, effects, and blow out, so I do not have a problem suggesting it to you. I know it works!

Would appreciate feedback from you as to what you do and what were your results.

Mark