Prop choice for Glastron CV-23

Ksgolfer2006

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Sep 14, 2014
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Current prop is 15.25 15P 3 blade. I can get about 38 MPH with WOT. Would love to get a little more speed but still have good hole shot. Motor is a 350 with 260 hp@4200. Alpha one drive. We plan to ski and tube with the boat. I was told to buy a 14.25. 21P. 4 blade. What are your thoughts. I'm new to boating so I have no idea and don't want to miss match my motor and prop.
 
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steelespike

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Apr 26, 2002
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19,069
My somewhat limited info indicates 260hp max rpm at 4400-4800 rpm.
Perhaps the hp is developed at 4200.Is your 4200 @ 38 mph at wot(wide open throttle),
or are you throttling down?
A very basic rule of thumb 1" of pitch change will result in "about 150 - 200 rpm change.
Increasing pitch decreases rpm decreasing pitch raises rpm.
Based on the max.rpm rating you should be shooting for about 4800 rpm at wot.
To me a 15" pitch should be over revving at wot. A 21 " prop is probably a more typical size but your numbers
€‹ don't support that change.
 
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Ksgolfer2006

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Sep 14, 2014
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WOT means wide open. There is no more. 4200 rpm is all there is. 38 mph at 4200 is all there is. Only me in the boat. Full tank of fuel. When I give it the gas it jumps out of the water and is on plane in nothing flat. Just looking for a different prop for a little more speed. The boat is in great shape for its age. Motor seems to be strong. Thanks for the info.
 

Maclin

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May 27, 2007
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6,761
I took the "260hp@4200" to be a spec on the engine, sorry.

IF the pitch really is 15, and IF the RPM's are accurate at 38mph, and IF the rpm's are 4200, then with 15% slip the drive ratio calcs to a 1.3. I am not sure that is an Alpha range, maybe for a big block but the Bravos came in for those later on.

IF the engine is at WOT and the boat is trimmed out and still no more rpm's are available then you need to go down on the pitch, but with a 15p already it would be very odd to have to go down to a 13p.

A typical setup for a boat like yours with a 5.7 would be a 1.5 ratio drive and 19p or 21p prop, and have a top speed of 50 or so.. But as mentioned by steelespaike the numbers you give do not really make sense.

I guess what I am coming up with is that, if all the numbers you gave are accurate, then even with a 15p your engine should be able to get to 4400-4600 even if it has a 1.3 ratio.
 

Ksgolfer2006

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Sep 14, 2014
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Thanks for all your help. Like I said I'm new to boating. If the info not enough I would be glad to answer any question you might ask. I'm sure I'm leaving part of the puzzle out for you guys. It's tough to give info on a boat that you know nothing about. The 15.25 and the 15p is stamped on the prop.
 
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steelespike

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Apr 26, 2002
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I think Maclin and I can agree that if all you're numbers are accurate and your only getting 38 @ 4200 there may be a motor issue of some sort.
A compression check is a good place top start.Note the location and condition of the plugs as you remove them.
Some other simple things to check. Is the timing set right and advancing as it should.Does the throttle open all the way.
assuming a 4 barrel Carb; do the secondaries open.
 

Tail_Gunner

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Jan 13, 2006
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6,237
fetch


Ok nice boat....ive acutally been looking for one for a while now yours looks like a winner from here. Based on your converstation with the starter you have a bit of to do's going on. One would be to check the plug's are they ac delcos and the right one's??? two would be to check the timing three would be check the tach accuracy there known liars very bad and need to be accurate across the entire rpm spectrum. Now why... im quite sure that boat should be running a 16/17 pitched prop at 4800 no problem's and as you already know boats are hacked together all the time......Is the speed gps???...Little side note its very easy to spend 1-500 dollars on a prop and get the wrong one based off your current number's.... it happens all the time and by doing this you will learn the important boat fundamentals.


ESSENTIAL DATA REQUIRED
When posting a question about propellers, you cannot get a reasonable answer unless you provide the following information in your post: Items 1-4 are essential for meaningful response to most questions.
1) Performance issue you are trying to correct.
2) Current prop manufacturer, model, aluminum or stainless as a minimum.
3) Current prop diameter and pitch (required).
4) Wide open throttle RPM and speed with an average load (very helpful)
5) Engine/drive make, model, year, and HP
6) Boat make model, year, length and weight

The below link would be a much more thorough list of data to provide if you're looking for detailed performance numbers,
http://forums.iboats.com/prop-questi...ml#post2336620
 
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Ksgolfer2006

Cadet
Joined
Sep 14, 2014
Messages
8
Well now that's funny. Looks just like the boat I bought. Thanks for all the help. I'll get it straight.
 
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