duo-prop size

Hello: I just became a member and have never posted anything before so if I make a mistake, I apoligize. Last year I bought the socorro. A 26' Blackman billfisher with a kad 32 4cyl. diesel engine. The boat is fantastic, It's everything I hoped it would be and more. I'm running a A-5 prop. Max throttle is 4000 rpm. I drop the rpm,s down to 3400rpm, full tank of fuel,bait tank w/bait, three passengers plus gear, I'm running about 18knots on calm water. I want to buy a spare prop. I've been reading about the A-4 prop and what I.ve been reading is that the A-4 is the prop of choice. If anyone has a opinion for or againest the A-4 versus the A-5, I would greatly appreciate your advice Thank you Jack pinto

Welcome John, you've come to the right place. You should have some of our resident experts weigh in on your question soon!

John, It's all about temperature, RPMs, and speed.

If you get 18knots at 3400 RPM out of a 4 cylinder with A-5 props, fully loaded, you are doing excellent.

Going down a prop size will lower your operating temp, lower your speed, at same RPM, or raise your RPM for same performance. What is your temp at 3400 RPM at 18 knots?

Hard to believe you are getting as good of performance as you are. Just buy a spare of the same size.. im my opinion. Want to try an A-4, I'll loan you a set, if it's not on the boat already.. heh

Hi Glenn (wizard) At 3400rpm my temp. is at 190. If I push it up to 3500rpm it will hit 200. Speed is a little more also. the engine has a super charger that puts me up on plane very fast. The engine and the 290 outdrive have 650 hrs on each. My friends are of the same thought as you are. They are impressed with the A-5 and they say I should not try to fix what ain't broke. Thank you very much for your input. Thank you from me and my boat Socorro.

I agree with Glenn but, it seems that the temp might be a little high at 3500 RPM's. I have the AD-41P and I run a little cooler at 3500 (about 190). I know it is a total different engine so probably nothing to worry about. Especially considering the weight of these boats when loaded with 3 passengers, gear, and full bait tank. I also do not push the boat past 3400 RPM's but, I feel that I can comfortably run at 3600 RPM's for some time without overheating. I'm running A-4's and they seem to be perfect. If Glenn is offering to let you borrow his A-4's it might be worth a little experimenting.

Good to see another Blackman owner posting. See ya on the water.

Scott (Tenacious)

Glen,
I would like to try your A4s
Phil

Hi guys: Glen (wizard) first of all, I would like to thank you for your most generous offer to let me borrow your A-4 props. I think what I'm going to do is buy the A-4s and since I have the A-5s already I'll experment with both and keep the one that works best on the boat and the other for the spare. And that way I can also show the same generousity as you have if someone has to decide between props. Second like Yellowfin1, I do not like to push my boat past 3400 rpm's. Even though my engine manuel says I should go to max throttle (4000rpm) then back off 300rpm. I get very nervous when the temp needle starts going beyond the 200 temp mark. Thanks Jack (socorro)

I have a KAD 42B, super and turbo, running A-5s. Runs like a banshee, on plane in 5 seconds, but stupid temp alarm starts going off at 185 degrees, 18 knots, fully loaded, now that the water temp avg is over 70 out there, annoying. Ran at 24 knots at 3400 RPM and never got over 180 in early spring when water temp was in the low 60s. I go up to 3400 RPM and it runs at 190 degrees. At 3800 RPM I can do over 26 knots, but temp goes to 200 too easy. Top speed ever ran was 31 knots for a few seconds, but temp runs away. Still trying to re-figure all this sh*t out again, since taking out my 42A motor and replacing with the 42B last winter. Solution: Disconnect alarm.. LOL


Still amazed you get almost the same performance out of a 4 cylinder that I get out of a 26' Billfisher that started with a 40 or 41, and now has a 42 that fits so tight, you have to take off the water pump to change the impeller.

I guess your making a lot more HP. LOL My Temp never goes over 160. I ran back in from the 182 today at 3300-3400 rpm running 20-21 knots with my A3s (1:1.78) gear ratio. Not sure I can swing A4s. Boat "dances" at 20 knots, sure would like to be able to run 24 knots, but thats not ging to happen. :rolleyes:
Phil