Ventilation problem Grew 178 GRS

[supercub1974]

Hi,
I have a new Grew 178 GRS bowrider with a yamaha F115 engine and am having trouble with ventilation. The boat bounces up on plane with three people in it with the engine all the way down but when up on plane and at anything above 1/2 throttle the engine "slips or spins out". The only way I can get to top speed of about 45 mph is to keep the engine trimmed all the way down until I get to about 40 mph and touch the trim up a little at a time until it reaches top speed, with the nose of the boat in the water and very close to being out of control like on rails. The motor was one hole from all the way down at first but I lowered it all the way with very little improvement. The AV plate is now about 1/2 inch above the keel with the engine in its lowest position. The prop is what came with the engine, a 21. Any advice would be great.

Thanks

 

[supercub1974]

Re: Ventilation problem Grew 178 GRS

The company "Grew" is gone out of business, can't get any advice from them.

 

[steelespike]

Re: Ventilation problem Grew 178 GRS

We need the information in the stickys at the top of the prop forum to tell if a pitch change is needed.
A 4 blade prop should help and usually it is suggested to drop an inch of pitch when going to a 4.
Is the prop in good shape?21" seems pretty high.

 

[supercub1974]

Re: Ventilation problem Grew 178 GRS

Sorry,
The prop is new with the engine, has no scratches or dents but I am unsure of the manufacturer. The only info i can see in the centre of the prop is "21K" and a K with a circle around it. I measured from the centre of the prop nut to the outside of one blade giving me a 6.5 inch radius, 13x21? The boat is 17'8" and made of fiberglass, V-hull. Not sure of the weight, its not in my info package I got from the dealer. When I did manage to get to full throttle with the nose of the boat in the water the gage was reading 6200RPM. I kept it there for only a couple of seconds due to instability of the boat and the fact that the engine had only 3 hours break in. The speed was maybe 45 mph.
So maybe a four blade will fix this?

 

[supercub1974]

Re: Ventilation problem Grew 178 GRS

Any advantages to a stainless steel prop?

 

[steelespike]

Re: Ventilation problem Grew 178 GRS

A stainless prop can give an overall improvement in performance and might resist venting,but 2 to 3 times the cost of an aluminum.
A turning point Hustler also might resist venting and give an overall improvement close to the ss prop at al. prices.
Then there is the 4 blade.Based on your 6200 rpm with the nose buried I think the same pitch would keep your rpm under control.
and resist venting,improve hole shot,stay on plane easier at reduced throttle,and improve control around the dock.
We don't know your true wot speed yet so I'll just say your wot will likely be slower than your ideal 3 blade.

 

[supercub1974]

Re: Ventilation problem Grew 178 GRS

Thanks for replying. Speed is not that important to me, just being able to get up on plane and be in control at 3/4 throttle is good for me. I would like to cruise at 3/4 throttle with the nose out of the water without having to think about ventilation. So I will try the four blade 13X21?

 

[steelespike]

Re: Ventilation problem Grew 178 GRS

Difficult to be accurate with the lack of a true wot performance. But properly trimmed up I would think could add at least a couple of hundred more rpm.The 21" 4 blade will knock 100-200 off the rpm.so should balance that out.

 

[supercub1974]

Re: Ventilation problem Grew 178 GRS

Just tried a friends 13 1/4 X 17 SS and the problem was the same but made the boat alot slower. I never went above 6000RPM and the max speed was 40MPH. I also found out that my prop is a 12 5/8 X 21, number was stamped in the back. Going back to the dealer tomorrow any ideas what to recommend?

 

[steelespike]

Re: Ventilation problem Grew 178 GRS

Usually a larger diameter prop will resist venting perhaps a 21" that's over 13 " would do better.

 

[supercub1974]

Re: Ventilation problem Grew 178 GRS

Thanks steelespike I will pass that info to the dealer and see what he has in a bigger diameter prop.

 

[supercub1974]

Re: Ventilation problem Grew 178 GRS

Just tried a 13 1/4 by 17 and the boat runs fine flat on the water at at full throttle (rpm a little too high at 6300) but try to lift the bow and it slows in and the rpm goes way up and i have to back off the throttle. Even if I can get enough bow lift to get spay from mid way down the boat would be okay but it just wants to lay flat.

 

[supercub1974]

Re: Ventilation problem Grew 178 GRS

Is there a brand of prop that helps lift the bow?

 

[supercub1974]

Re: Ventilation problem Grew 178 GRS

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[supercub1974]

Re: Ventilation problem Grew 178 GRS

Thank you Steelespike,
I bought a 13.25 X 19 Solas prop and all problems are solved. Hard to believe the wrong prop can cause so much trouble. Thanks again you are definitely a supreme mariner.