Prop Size...Suggestion

[Islandtrader]

While I am waiting for the dealer to come up with some suggestions I thought I would see what you folks think.

Boat wt. approx. 3800#

Bravo 1 w/1.65r

383-350 hp. wot 4800/5200

Cruising? guessing 3200-3600

So what size shape and form are folks using out there.

[76Red18]

According to a prop slip calc and Crouch's speed calc a 23 pitch with 9% slip @ 5000rpm = 61 or so... She's gonna fly!!
Prop Gods might be of use with this one since there isn't much data on the 21.

[Fr. Frank]

Assuming average high-performance SS propeller slip of 12%-15% at WOT, with, say, a 22"P Trophy prop by Quicksilver, you're looking a a top speed at 5000 rpms of around 54-55 mph. That assumes, of course, that you have the power to turn that prop at that rpm with that weight.

If you have to drop to a 20"P prop, your WOT speed will be 48-50 mph, assuming the 12%-15% slip.

Prop slip tends to increase with lower speed, so at the given rpms figure a cruise speed with the same 22"P prop of 32-35 mph, and with a 20"P prop of 31-33 mph.

Things like drive angle, height of cavitation plate above the kelson, hull smoothness, amount of rocker or hook in the hull, etcetera, all affect the efficiency of the prop.

A slip of under 10% at WOT is possible, but rarely achieved without mods like nosecones, carefully adjusting drive height and trim angle.

I was able to get a slip ratio of between 8% and 9% when I had my 2.0L Merc XR4 V6 on my Seafari, with slick racing paint designed for sailboat hulls, a Bob's nosecone and low water pickup, motor mounted pretty high, and a very expensive, custom-made, one-of-a-kind propeller made for me by PowerTech.

If you did achieve a slip ratio of 8% at WOT, then your top speed at 5000 rpms would be 58 mph with the 22"P prop, and 52 mph with the 20"P prop.

[Bushwacker]

Terry,

You might send an e-mail to Bob, the skipper of the SeaCraft 21 "Unohu". He had the full bunks and the big f/g seats, so may have been a little heavier than yours. The last engine he had in it was a 270 hp/350 Chev/Volvo outdrive. He said that was a 50 mph rig, so I'd think you could run mid-upper 50's or maybe lower 60's with the extra power you have. I've forwarded some of his e-mails to you so you should have his address, but let me know if you need it. Denny

[Islandtrader]

Speed will take care of itself

I am a little at lost on the number thing.

What size prop and what pitch

[Fr. Frank]

Speed will take care of itself I am a little at lost on the number thing.
What size prop and what pitch

It depends on your motor, terry. You want to have the "largest" pitch prop that you can turn and still reach 5200 rpms while fully loaded.

When I was chosing props for Wellcraft's Scarab division, we would load the boat to the maximum recommended load, and then find the prop that gave us the best performance while still able to turn the maximum rpms for that motor. We call that "propping for max of max at max". That's being able to turn maximum of the maximum rpm range while loaded at the maximum weight.

It would be helpful to know the hull weight and maximum load for the hull. I'm going to use the numbers from the old early 70's 21' Regal cuddy to start with. From that I can try to figure you're starting point for propping.

Hull weight (no power): 2175 lbs
Powertrain weight: (383/350, B1 drive) 993 lbs
Fuel weight (100 gallons): 630 lbs
8 persons @ 150 lbs ea.: 1200 lbs
Miscellaneous equipment: 300 lbs
Provisional max gross weight: 5298 lbs

For a starting point, in a Mercury Quicksilver prop, try either the Rev 4 14 5/8"D X 19"P 4-blade, or Mirage Plus 14 3/4"D X 21"P 3-blade, or the Enertia 14"D X 19"P

[Islandtrader]

Thanks,

Good numbers...my x-dimension is approx. 7 inches. So with that in mind I think if I only can try one prop at a time I will go with the Mirage...3 blade.

What do you think?

That is a lot of max weight I was thinking 1000# less. But what the heck

[76Red18]

Hey Terry I have an old mirage 17 you can try. No where enough pitch but you can see how it carries the hull. You'll need a hub kit. I'll meet you half way. Takes me about 45 minutes to get to 41 and 776. 239-565-9328.

[Fr. Frank]

Personally, I love the Mirage Plus. It may be one of the 2 or 3 best stern-lifting props out there, and is probably the #2 in a 3-blade.

If you install that 17"P, be careful not to seriously over-rev your motor. It should jump on plane, but at max of 5200 rpms, you're likely to get about 45-47 mph. And I promise you, that's still flying!

[Islandtrader]

Hey Terry I have an old mirage 17 you can try. No where enough pitch but you can see how it carries the hull. You'll need a hub kit. I'll meet you half way. Takes me about 45 minutes to get to 41 and 776. 239-565-9328.

Rich, Thanks.I may take you up on this...however I am waiting for a callback from BBlades to see what they think before I go making a decision.