Scarab Sprint Prop Help Needed
 

I am trying to find a better prop for my Scarab to help get a good top end without totally killing the great hull shot I have currently. I am current running a 19P SS Merc Vengence (i/o) 3 blade and hit redline at about 2/3 throttle. This keeps the top end about 48mph.

From using a prop calculator I am figuring the prop slip to be about 0.16. Does this seem reasonable for a stock Merc prop?

I found a great deal on a 24P 4 blade SS prop, but am worried that this might be too much prop to get out of the water. The calculator says this will put the top end at about 60mph assuming the same slip at redline. Would the 4th blade make that big a difference if I make this big of a jump in pitch? Is there anyway to calculate the effect on getting to plane? Any thoughts?

Barometer Soup

I think that you can get much better performace from your boat, than what you are getting now. First. What kind of power and drive, are you running? Second. the Vengance is in it self, a very poor hi-perf prop, and going to fx. a Mercury Mirage Plus, would be a much better choice. The 16% slip is somewhat high, especially if your drive sits low in the water. 10% would be good on your boat.

It would be easier to guide you, if we knew your drive hight. Trim your drive so the prop shaft and plate above, is parallel wilt the bottom. Meashure the distance from the bottom to the prop shaft centerline at a 90 degree angle on the bottom. I find that the easiest thing is to meashure from the bottom to the underside of the anti ventilation plate, and then from that to the prop shaft centerline, and then subtract the difference. If the drive is very low, and the prop shaft c/l is 6½" - 7+" below the bottom, go with a three blade if higher, I would think that a four blade would be best. What ever you decide to do, get rid of the Vengance. Try a 23"p Mirage Plus. If the pitch is too low, go higher and so on. You should also try the Revolution 4 23"p and the Bravo 4 24"p. I think that a good 23"p prop will go for about 60-65 mph with a 1:1.5 ratio, depending on the engine. If you run a Mercruiser black motor, they usually have max power between 4700 and 4900 rpm.

I have a 23 pitch Turbo Prop that I would sell you, It was the fastest prop on my boat(59mph), and what the Donzi guys like to run, but my hull likes the Mirage better. I was keeping it as a spare, but I have 4 or 5 props, and its tax time, so I will sell it if you think its something that you could use, 24 4 blade is a big jump from a 19 3 blade, I would guess its more along the lines of a 23 3 blade, or 22 4 blade that you need.

A.O. Razor - Thanks for the information. I'll check the drive height when I pull the current prop in the next couple of weeks when I get ready for the season (It's still snowing here in Michigan).

I am currently running a 4.3L with an Alpha Drive. It's the smaller HP carberated version, but it doesn't take much to move an 1800lb Scarab so it still gets on pretty well. I think its about a 1.62 ratio from what I am current calculating. Will a Mirage plus work on an Alpha Gen II?

rchevelle71 - What do you want for the 23P Turbo? PM me if you get a chance. Thanks.

With that power and ratio, 23"p should be in the area.

If you are only running a alpha drive on a 4.3, I would try the mirage first. A prop like the Laser 2, might even work. The cool thing about the Laser 2 is, that the blades are thin, and it,s relativly small in dia. but it has good bow lift. If it can get your boat on plane and carry it, I think it would give you the best top end. I would stay away from the Bravo prop. It will be too big to spin.

I have had good results with a Laser 2 on a Stingray 23 SX and a 496 HO. It worked better than the Mirage despite it being a 23' boat. It might not be the prop you hear about mostly on this forum, but it has been around for years, works well and is fast on smaller boats. But again, you will have to try.

We have a complete line of test propellers and can dial you in quite easily if you would like.

Call us at you leisure and we can discuss your set up in depth.

Brett

Okay, I checked the drive height (center of prop shaft to the hull bottom) is about 8". Also got my hands on two 3-blade Laser II's, one on loan and one I plan to buy (20P and 23P).

When the boat gets in the water in the next month or so I'm going to see how the Laser II's do, and I may have them tweaked to get them dialed in right.

Brett - I left you a message with Susan last week so hopefully you've got all the information you need. Just let me know if I missed something. Thanks.

Sounds like an idea:drink:
Keep posting, It'll be fun to hear what you end up with for prop, and how much speed and performance you will gain. Should be quite a difference:cool:

I did recieve the info. I'm just a little behind on my call backs. I will be caught up in a day or two.

Thanks,
Brett

Okay. . . I ran the 23P Laser II and it was quite a difference. The time-to-plane was 1-2 seconds slower (with a full tank and 2 adults), but I gained around 11mph from the Vengence. The Laser ran about 58-59mph with a slight chop on the lake but the boat liked this prop much better (still need to run it with my GPS).

Definitely better bow lift, and I noticed that I needed much less trim at speed than the previous prop. Cruising speed felt quite a bit better.

When I loaded the boat (slightly beyond max weight), it slowed up quite a bit getting out of the water (5-6 seconds)but also felt better in the mid-range RPM.

Amazingly, the 23P at redline still had throttle to spare if I would have had more pitch. No doubt with the right prop this would break 60. At this point I'll start experimenting with 4-blade, trim tabs, pitch tweaks, etc. to see if I can balance the hull-shot and top end.