Gannz

21’ Bow rider. 3000 lb dry and I guesstimate a normal load of 750-850 lb of passengers, gear, and liquids. Not an offshore boat but y'all know the most about performance upgrades.

350 Vortec, 4 bolt main, LT4 Hot Cam & springs w/ 1.5 rockers, Summit Dual Plane Aluminum Intake, Edelbrock 600cfm 4 barrel
Maybe 300HP?

1.65 Alpha One with a 14-1/4 x 19 SS prop and a Hydrofoil

It did have a 305 stock, pre-vortec in it with the 19 pitch prop. Since the upgrade, I’ve only run WOT two times for short bursts to check the RPM and top speed which was 5600 RPM and 54 MPH (clocked on GPS).

I rarely run WOT. 95% of the time we’re on a decent sized, busy lake and the chop it too rough to run WOT. At least for the wife. LOL. At dusk when the water is like glass I can run WOT. I tried to build the motor for low RPM torque because that’s what’s most useful for us. I prefer a good hole shot and we pull the kids tubing more than I run WOT.

I know I can just back off the throttle to keep the RPM down, and I do, but I have the extra RPM to go up in pitch and gain some more torque. Also, if I have one complaint about the boat, she’s stern heavy. I think maybe that’s why the original owner put the hydrofoil on her. Normally, the wife & I are in the captain’s chairs and the kids are in the bow and she runs great. Trim is between all the way down (heavier chop) or 2/3 up (glass). Only problem I have is when we add 2 adults to the back seating. Can’t trim it down far enough to keep the bow down so I can’t run above 3200 or so RPM without porpoising and she corners awful. Even at minimum RPM to stay on plane the bow is too high.

So, I’m thinking a 4 blade prop to get more stern lift and torque and 23 pitch to get the WOT RPM’s down to where they should be. I wish testing various props was an option but there’s no one around me that would do that and I don’t want to blow a grand or more trying to find the best prop.

Advise on what size prop I should get? 3 blade or 4 blade?

Or maybe a 23 or 25 pitch 3 blade and trim tabs? The stern heavy issue is only a problem on the rare occasion we have more than 4 passengers so I don’t want to spend 500+ for a set of electric or hydraulic tab. If those self leveling ones are any good it would only cost me about as much or a little more for a set of those and a 3 blade prop vs a 4 blade prop and no tabs.

94sylvan

I just replaced a 220 hp 305 with a 375 hp 383 in my 21' BR. My boat weighs in about 400lbs. heavier and I was between a 17 and 19" 3 blade previously. I also am stern heavy so I added Nauticus trim tabs. I have started with a 21" 4 blade and even though I'm still working through some carb and engine issues, I can tell you I didn't jump up enough in prop yet. I'm trying to prop for 5000 with light load so with a full boat I'm still in the 46-4800 range. I'll end up around a 25 3 blade for top end when done it seems. I do like the Nauticus tabs and the 4 blade out of the hole though! Major improvement for minimal investment.

94sylvan

I forgot to mention, I'm at 1.5 ratio rather than a 1.65

Trash

Where does you motor make its peak HP? If you load it with too much prop you won't be able reach peak HP and will be slower and low end performance will be doggy.

I would ditch the hydrofoil in the interim while selecting props. 21" Laser II or perhaps a 21" Mirage would be a good starting point.

Gannz

Thanks for the info. Nice upgrade on the motor. I considered a 383 but didn't want to risk tearing up the Alpha One or have to lower the gears.

I punch the numbers into the mercruiser prop selector and it says I should go with a 21 pitch and I'll get 62 MPH at a max 5000 RPM. At -200 RPM per 1" increase that won't get me to the 5000 and 62 MPH seems a little optimistic.

Gannz

I think 4800 RPM with a light load and driver only?

I did a little more testing last night and I was pushing about 5700 RPM with just driver and average load. Top speed was also down to about 52 because I couldn't get the trim high enough, long enough to build up speed without porpoising. Having the extra passenger weight in the bow (about 200 lbs) really helped. Won't it be worse without the hydrofoil?

If I go to a 23 pitch that should put the RPM's at 4800-4900. If I had 1.5 gears I'd go for a 21 pitch but with the 1.65 I'm really leaning toward the 23. I wish I could just test about 3 or 4 of them but I can't. No one around me has a loaner/test program.

Should I ditch the hydrofoil and do some more testing? I'm going to hold off on any tabs until I get the prop right.

94sylvan

I really think you would like the 4 blade tubing and for hole shot, especially with the rough water most of the time. I think a 23" 4 blade will be too tall though for your setup. You could always do an aluminum prop to test and not have alot of money tied up when compared to stainless.

Gannz

Punching some more numbers into the prop selector and it looks like I'm right on the borderline between a 21 and 23. With the 5600 RPM and 54 MPH it put me in a 21 P and calculated I'll be at 5000 RPM and 62 MPH. With the 5700 RPM and 52 MPH I got last night (lighter load by 3 passengers) it recommends a 23 P and calculated 4900 RPM and 63 MPH.

I know that's all theoretical but that, and everyone's real world experience with similar specs to mine, is all I have right now. Just trying to nail down the best place to start since I can't test them. I'm thinking of buying used on fleabay or the forum and if it doesn't perform well I can sell it for about the same as what I paid.

Also reading that the Mirage and Laser II have a higher rake which is going to lift the bow. That's the last thing I need. LOL. I wish I could ride at WOT with the bow higher but I can't go above 50% with a light load and calm water or about 66% with 2 passengers in the bow without porpoising.

Gannz

I'm really torn. LOL I hate these kind of decisions. Same thing happened when I was building the 350. I thought I was happy but now you have me wondering if I should have stroked it. LOL

My hole shot is really pretty good even with the 3 blade prop. I know I said I'd want more but now I'm not so sure. I've never timed it but I'd estimate she's on plane in about 2 seconds when not tubing. And I've torn a canvas tube in half by gunning it out of the hole. It might have just been a POS but now I gradually increase RPM's when starting off with the new, and same brand tube. LOL

And the waters really not that rough. It's just too rough for WOT unless it's at dusk. I cruise around fine at about 40-66% trim and 3000-4000 RPM. I can usually maintain RPM's and just trim down if she starts porpoising or I have to run into a wake.

phragle

Are you porpoising because your prop is not carrying the bow where you want it?