Why Is My Boat Slow

[cutlas]

I may be wrong but I feel my boat should have more top speed. Its a 89 vip vindacator with a 7.5 340hp woth a king cobra outdrive. Its 23ft wieghs 4000 with me and one other in the boat it will run 52mph(gps) . I can run from 4400 up the 4800 with no speep difference. I have a 14.24x 23 viper prop... does this seam right ?

my buddy has a 23 ft rinker with a 300hp with a 21 pro and he runs 58-62

 

[matthewpa]

Re: Why Is My Boat Slow

Your problem is the 23p prop. You need to prop down to a 21p for that boat and engine combination. Buddy of mine has a similiar setup with only 300hp and tried my 21p on his boat. He noted the same issue, no speed increase from 4400 to 4800 rpm and he actually dropped 7mph on GPS with the 21p vs. the original 19p he had on the boat.
Switch out to a 21p.
A great site and great props can be found at....
NO Direct links to competition...we sell those props right here at iboats

 

[QC]

Re: Why Is My Boat Slow

I don't think I agree with above ^^^^ . . . yet. Not sure why your buddy's situation is comparable as they are all different, props, hull, power package, trim etc.

Let's get a little more info and then we'll see if there are any quick fixes. My first thoughts were 1) 52 isn't all that bad and 2) how does your boat react to trim? If it is not making more speed from 400 more RPM then slip is rising dramatically. I also doubt it doesn't go up at all. Are you saying that you see 52 MPH on GPS at 4400 RPM and then you hammer the throttle and get 400 more RPM and absolutely NO increase past the 52? Could be blowout, which maybe it is trimmed too far out. Can you make her porpoise? What happens when you trim more? Less?

BTW, welcome to iboats!!!

 

[cutlas]

Re: Why Is My Boat Slow

What is a avg increase with a pro change like this ? I kow the ss props are exspensive.. So I hate to spend the money for say 2 mph..

 

[cutlas]

Re: Why Is My Boat Slow

ok here is what i did . we have miles of straight calm water I came up on plain and went wide open . then i tap the trim ever few seconds and look for speed increase. and rpm.. at 4500 rpm gps said 52 I tapped and rpm went up slightly but mph stayed teetering between 52-53 as i went up with trim rpm increased and mph stayed until I raised to far then rpm went higher and speed decreased.

 

[matthewpa]

Re: Why Is My Boat Slow

So you are at the low end of your RPM range with that prop and it is aluminum. What type of prop do you have on there now, standard factory sluminum? Do you have the numbers off the prop?

 

[cutlas]

Re: Why Is My Boat Slow

no it is a stainless steal viper 14.25 x23

 

[TilliamWe]

Re: Why Is My Boat Slow

And what is the WOT RPM range for that motor?

 

[cutlas]

Re: Why Is My Boat Slow

4400-4800

 

[codertimt]

Re: Why Is My Boat Slow

And what is the gear ratio for the King Cobra? I've seen it could possibly be 1.42 and 1.63...

 

[QC]

Re: Why Is My Boat Slow

Cutlas,

I think you are very close actually, but you are starting to "blow out" before you reach the top of your trim limit. I would try and move some weight aft. Take anything heavy from the bow and shift it to the stern. What you want is for the boat to start to porpoise just at the point of highest trim. If then you can control the porpoising by bumping the trim one click down you are as close as you're going to get without trim tabs. My guess is you could gain 2 - 3 MPH this way. Maybe more. You are already within your WOT operating range, so I would not change pitch until you do a little more testing. BTW, 2 MPH is a big deal in boat speed tweaking . . .

60 MPH boats are few and far between except with mega power and light OBs like Bass boats. I think you are very close. Also, with an older boat you may have some extra weight on board. Some you may be able to simply remove, but it is possible you have some soaked flotation or other water log type condition. EVERYTHING matters. Good luck!

 

[cutlas]

Re: Why Is My Boat Slow

ok Thanks Ill stay with what I got... Thanks for the Help to all who replied....

 

[Dhadley]

Re: Why Is My Boat Slow

Food for thought - that drive has a pretty large gearcase diameter. That prop leaves a lot to be desired. If it were me, I'd look into other props with more diameter and/or ones with more lift.

That's a great drive, it never gets the credit it deserves. However, the next generation King drive was slightly faster.

 

[cutlas]

Re: Why Is My Boat Slow

well yester day when i started this topic I sent a email to the local prop shop . this is what I got back from them.

A 3 blade Lexor in a 25p would probably do better on top end.
This prop is 14-1/2 x 25. If you are pulling a Viper 23p 4400 - 4500, then you will probably pull the lexor 4200 - 4300.
This prop is $375.00 and is made by Solas.
I do not have this prop in stock, it would probably take me 3 - 4 days to get one.

 

[QC]

Re: Why Is My Boat Slow

But that ^^^^^ would be below your recommended operating range.

The interesting thing is that a guy at the beginning of this thread recommended that you go to a 21P without having figured out why you are blowing out. The prop shop also recommends that you add pitch also not knowing why you are blowing out.

The fact is that neither of them know until you get some more info. and neither do I. I am not recommending that you keep it as is, but you need lots of info to make a sure prop selection and with what we have here either of those props could actually be slower. I will recommend that you listen to Dhadley as he knows his ship. More bow lift as he suggests should translate into more positive trim which is another way of saying more trim before it blows out. With the info that is here, THAT is a solid plan . . .

I think I would play more with what you have. Shifting weight around to learn what is, and what isn't, is where I always start . . .

 

[cutlas]

Re: Why Is My Boat Slow

well being that i dont have alot of time on this boat. Im not gonna run out and spend $300-$400 on a prop til I know for sure I need or can benifit from it....So I will use my current prop however I have no wieght upfront to shift other then a radio and two speakers in the cuddy wich wieght about 2lbs....