383 / Alpha prop selection
 

Have a 21' Caravelle Interceptor, approx 3000#, with a new 383 (Dyno @ 340 Hp / 395 ft lb) and Alpha Gen 2. Existing prop is stock 21P Aluminum (orig engine was 5.7l / 250Hp) which will blow past max rpm if allowed. Trying to decide which prop to use. Am pretty conservative and follow all the general 383 / Alpha advice.
Do not know current WOT (should I try with the 21p?). Do want to use Flo-Troq / rubber hub. Like the idea of 4 blades (but don't want to fry the Alpha). Mostly family/board/ski boat, but do not want to totally kill top end either.
Looking at vented Mirage plus, Laser II, Revolution, etc. Also figure should require around 24 - 25 Pitch, but would I be better off going with smaller pitch (3 Blade?) and adding cup, or larger pitch and labbing if the max RPMs are slightly off? Also if prop work is needed, who could help out? ==> sounds like Houston prop is out of the picture...

Any advise appreciated.

Re: 383 / Alpha prop selection
 

At what rpm is max torque and HP? This will play a major factor in deciding what pitch you need to run.

I would try the stock prop with you new motor. Keep an eye on the rpm. The dyno sheet will give you an idea where you turn the motor at wide open throttle.

A 4 blade would be an option as a good all around prop yet still maintain top end speed. I would go with the plastic flo toque insert in your situation.

What was you top speed and at what rpm with the stock engine.

Matt

Re: 383 / Alpha prop selection
 

With the exact weight of the boat (boat, people, gear, fuel, water, etc.) and your desired wide open throttle rpm, I can give you the correct propeller pitch. At least get close and we will have something to work with.

I like the Revolution 4 prop on hulls like your for all around performance. If you are looking for top speed, the Mirage Plus can be Labbed to get the best number. Your boat is on the border line as far as the laser 2's capabilities and would probably stay away from that one.

You can't get the rubber hub Flo-T hubs in any prop Merc will make for your boat, so the delrin sleeve will come automatically. With your hp and T, I,d still be considering the solid hub. Otherwise, keep spare inserts on board so as not ruin a Saturday.

We can help further as you get more info.

Re: 383 / Alpha prop selection
 

The rubber hub IS an option for you if you really want it. There are rubber hubs designed specifically for the Props with a square bore. On an occasion we have a customer who wants the shock protection of a rubber hub and we will install it in the square bore props.

Matt

Re: 383 / Alpha prop selection
 

Actually we designed the square rubber hubs back in the early 90's to fit into the round bore props to prevent the premature slippage from rehubbing. Some other manufacturers now also offer square rubber hubs Yes they will fit in the square bore.

The point I was making is........you will only get the delrin sleeve when you purchase a prop for johns boat.

Re: 383 / Alpha prop selection
 

Close est on boat weight ==> 3400# loaded + 36 Gal Max fuel.
Max Dyno Hp is 343 @ 5100 rpm. Max torque 396 ft lb @ 3000 rpm, though torque holds above 380+ ft lb across curve (3000 to 4500 rpm). At 5100 rpm torque is 360 ft lbs. Also, the "corrected" power curve ups all data by approx 20 Hp & 20 ft lbs across curve. Original 5.7l merc cover indicates max operation rpm @ 4900 (probably want to stay around this number??).
The Interceptor boat design tends to be sport oriented (step hull, pad in rear...). From what understand it ran just shy of 60 with the 5.7l & 21p.

Sounds like either a three of four flade would work well. Like the low / mid performance of the four (Revolution 4 or Vensura ???), but really do not want to trash the alpha (this is one reason did not max out the 383). Also like the performance side of the Mirage Plus.

Do you think would be ok with the four blade & alpha if maintain conservative operation? Really leaning toward the cushioned hub for additional drive protection. Also, can you supply the prop / hub, or would I contact just for service?


I know this has got somewhat long. Am new at all this, and really do appreciate all the help & direction.

Re: 383 / Alpha prop selection
 

I'd expect a top speed just shy of 70 with the new power.

You shouldn't have a problem with a 4 blade prop on the boat. If you plan to fly the boat in rough water then you may want to re-think the 4 blade.

A 24 pitch prop should get the rpm about 5100. Remember the higher the wide open throttle RPM the less torque there is on the drive. We can install a rubber hub in a new prop (at no charge) or an older prop if you prefer the rubber hub over the Flo Torque system. The rubber hub provides greater shock absorption when shifting gears over the Flo Torque hub system.

On your application a Rev 4 is a good option for a 4 blade. Personally I am not a fan of the Vensura (formerly know as the Offshore 4) as I have found it has a poor top end capability.

Are you sure about the RPM at which max torque is achieved? If so you should have no problem propping the boat out in the high 4000's or low 5000's. You will find the boat should have ample pull to get it on plane with the family on board and a skier in tow.

Matt

Re: 383 / Alpha prop selection
 

Pulled HP & Torque are off of print out. Used Owens racing near Houston & witnessed pull, so feel pretty good about the numbers.
If go with the Rev 4, should I just buy it? OR whould it be better to borrow any 24P first to make certain a 14 x 24P comes close to desired WOT. If just purchase Rev 4, can it be labbed, or trailing edges cupped if the WOT rpm is off?
Want to get this close to correct first time. Wife would not understand multiple SS prop purchases just for testing.

Thank You.

Re: 383 / Alpha prop selection
 

I also don't care for the Ven-Sura on your application.

The Revolution 4 propeller comes in odd sizes to 25 pitch. We do have test propellers available in all pitches

Re: 383 / Alpha prop selection
 

If you know someone who has a prop you can borrow that works well. If someone offers you a prop to try and it's about the right pitch try it! Depending on where your rev limiter is set you may be able to try a 23 pitch prop. If it goes to 5300 and you want to run at 5100 then we know, within reason, that you need a 24.

We offer test props and of course will exchange a prop if it's not the right one. If you don't know for sure what prop and pitch you need be careful where you buy as many places have a no return policy.

As far as lab finishing and any other prop work that's up to you. If you plan on having the prop lab finished you would want to prop on the low side of you rpm range. That way if you have your prop lab finished you can afford the extra 200-300 rpm that a typical lab finish will give you. There are many adjustments that can be done to a prop to change it's characteristics.

Typically after a customer runs a prop we discuss how the prop reacted on their boat and find out what the customers goals are. Then we make a recommendation what we think is the best service, or none at all, would meet the customers goals.

Matt
Throttle Up Propellers
727 531 2255