Another random prop question.

[thejetman]

Heres the deal. 20" Wellcraft Excel, she weighs a hefty 3300 lbs. Its mostly used for cruising. Specs:
Late model 4.3 with Vortec heads, 14.5 x 19 prop on it.
I have been doing some research and found out that the lower unit is a 1.61:1 ratio.....
Here is the question, why does it do 45 mph at wot which is at 6000 rpm? The boat runs great but seems to be under propped. If I go from my current 19 to lets say a 23 will It bring my cruise speed up and bring my rpms down?
The hole shot is awesome right now but it seems as like I could get more top end from her. I am not really concerned with towing stuff and what not.
I really want to improve the midrange to top end, she planes in 3 seconds flat right now.
Suggestions please?

 

[Mac525]

Re: Another random prop question.

sounds like you've got the right idea to me you could stand to lose 600 to 800 rpm

 

[craze1cars]

Re: Another random prop question.

I disagree. One of your numbers provided has to be wrong.

Use a prop slip calculator and play with it a little bit. If you are turning a 19 pitch prop at 6,000 rpms on a 1.61 outdrive ratio, your theoretical top speed should be 67 mph. If your actual top speed is only 45, then you have 33% slip. Unless your prop is practically missing an entire blade, this is not realistic at all on a planing boat.

Before changing anything, confirm your speed with GPS, confirm your tach with another known good tach, double-check your outdrive ratio, and double-check your prop pitch. Is it possible your prop has been modified/repitched/damaged? Is it possible your prop hub or engine coupler is slipping?

Also if you are truly running 6,000 rpms, I believe your engine, and probably outdrive, will live a VERY very short life. This is an extreme over-rev for an I/O....

For reference, my Vortec boat planes out in about 2.7 seconds with a 19 pitch prop that I use for heavy load days (it's a heavily cupped 4 blade so you add 1 to 2 inches of pitch for cacluation purposes)...so your strong holeshot is nothing out of the ordinary. With that particular prop on a light load, my boat maxes at 5,200 rpm, which is also a bit of an overrev. I have a 1.60 drive ratio like yours. And my top speed with that prop is 57.5 mph.

So why can I run the same ratio you have, at 800 less rpms than you, yet get 12.5 more mph than you? Again, unless about 25% of your prop is completely missing it is simply not possible....which is why I very much believe you have an error in your figures.

 

[thejetman]

Re: Another random prop question.

Well I would love to check every thing but Im on a limited budget, so getting another tach is out of the question. I dont have a GPS, I am just going by what I see on the gauges. Its my first boat, and its only been out twice. I am just trying to get the most out of her. I might be wrong about the gear ratio and the tach and speedo may be wrong. Just lookin for info.

 

[Mac525]

Re: Another random prop question.

Is the possibilty that your prop has the wrong hub size and it ventilating the prop with exhaust gas this would cause all the simptoms you are having If your numbers are right. you can get gps app for most cell phones, or maybe find a friend with one. Can you post a picture of the prop on the leg?? There are a lot of ifs here.

 

[thejetman]

Re: Another random prop question.

Well, I pulled it all apart last weekend and found that I was missing the spacer that goes in front of the prop. Ordered it this morning. As far as ventilating goes......I have no idea what you are talking about. I asked the guy at the local prop shop this morning and he recommended a 21p......I am kinda broke right now so if anyone has one for cheap let me know!

 

[Tail_Gunner]

Re: Another random prop question.

If you cant get a tach read then a 21 in a solas 3 blade right here @ i-boat is a great prop and that should be very close for your boat....i have one. But take a flash light and look at the back of your tach...there should be a switch with these selectable settings 4-6-8 make sure its on 6...and for good measure move the switch back and fourth a few times...and make sure again it is on 6..Good luck
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