Noob, not sure what to buy.

[MCNPathfinder]

Good morning! Pardon my ignorance. I have what seems to be somewhat of a unique situation. I bought my boat a few years ago from a friend. Long story short, my brother and I ruined our 4 blade prop and lost it, so I'm not sure what we had. We've been running 3 blade props, but it doesn't have the torque to ski/wakeboard, etc.

It's a 1990 Cobia Spirit 2250CC. I believe it has the Alpha One drive with the 5.0L Mercruiser. The friend I bought it from said it's a built 5.0, so should be right around 300HP. Whether that's true or not, not really sure. iBoats says 250HP top end. From what I recall it seems like WOT was somewhere in the 4500-5000 range, which lines up with iBoats. It seems like I was somewhere around 35MPH via GPS when I measured.

I'd like to go a little faster, but need to be able to get the torque for wakeboarding and pulling skiers out of the water. Oh, and getting on plane better than this 3 blade. I'm not entirely sure that what I had was right for it to begin with.

Can you help? Any recommendations?

Thank you!

 

[Sea Rider]

Which rpm was that motor turning when running at 35 MPH ? assume full throttle ? knowing actual 3 blade size and pitch could maximize a prop to match what you want to accomplish. If going blindly on a 4 blade, whichever you choose will be your starter prop, test it and later maximize it with another 4 blade for your skiing requirements. When skiing, pulling inflatable toys is it while boat is lightly, medium or fully loaded ?

Happy Boating

 

[MCNPathfinder]

It was a while ago, but I think it was running like 4600. I'd obviously like to be kind of conservative with the new one as I don't want to risk wrecking anything. I want to say my 3 blade has an 11" pitch, and we're generally medium loaded when we ski/tube.

 

[Sea Rider]

If skiing/pulling toys while medium loaded achieved 4600 wot revs with a 3 blade 11 prop pitch, must go down 2 less size pitch to a 9 to pull wot revs to max 5K. If boating medium loaded say family oriented is a must do, bear in mind that when lightly loaded wot revs will go higher, so throttle less and check revs with tach.

4 blade prop is other boating story as you don't know previous one which size and pitch was as to buy same if was a good performer.

Happy Boating

 

[SkiDad]

something doesn't add up - 35 is really slow for a V8 - are you getting spark on all cylinders ? That kind of power should pop you up on wakeboard with almost any prop.

I would say you need to see where your 3 blade is putting you rpm and speed wise and report back. And look up the pitch of that 3 blade.

 

[Starcraft5834]

Assuming that engine is healthy, I highly doubt you have a 11 pitch on that drive.. unless the boat is super heavy and water logged. (possible) .. look on the collar of the prop, should be bunch of stamped numbers there look for "-" at end with # there.. that's the pitch of the prop... cant really recommend much until we know what your running and get WOT- rpm #'s back too

 

[MCNPathfinder]

Thank you for your help this far! It's quite possible the boat is water logged. It has some soft spots in the floor, or just one really. However, the 35 at 4500 was with the 4 blade prop. I guess I haven't measured with the 3 blade. Guess I need to find that old prop if I can and get the numbers off my current.

 

[harleyman1975]

Just food for thought here...I am running a 14.25" diameter x 19 pitch 4 blade aluminum quicksilver prop on a 20 ft regal with a 305 2bbl and am able to turn 4850 RPM's with 3 ppl in boat (48 mph on the pitot speedometer). Planes out right now! Less than 4 seconds from dead stop to plane. I can only make 4600 RPM's with a 3 blade 21" aluminum quicksilver. So your boat is a little heavier but if your engine is really 80 more horsepower than mine then you should be pretty comparable. Many prop shops will let you try different propellers as long as you bring them back in perfect condition.

 

[JonBrown]

I think when he said 11' prop he is measuring circumference and not giving you any pitch degree. He needs the numbers off of the prop and definitely a true and accurate no guessing number of RPM per gps mile . And then re confirm RPM to gps mph to trial another prop. You can gain tourqe from the 3 blade or loose it and speed from the 4 . You will need the right prop and numbers tell where to go . If the boat normaly will hold 3 souls and a skier. That is what you test with.
My 2.5 cents worth .. Hope it helps.
jon

 

[Sea Rider]

Plan finding the old beat up 4 blade prop to check stamped numbers on body, right ? Your actual current prop is a 3 blade with which pitch ? If so, could maximize a less prop pitch if in need to pull your wot revs up while boating at max wot rpm range as currently loaded.

Happy Boating

 

[Silvertip]

Sounds like you want your cake and eat it as well. In your original post you indicated current rpm is about where it should be but you want to go faster. Without exceeding WOT rpm recommendations, that can only happen with a prop with more pitch. Unless the engine has the power to twist that prop at the same rpm you will not gain speed but may in fact lose speed. You will definitely lose hole shot/on plane time because of it which is also something you also complained about. Problems getting on-plane requires a drop in pitch (gearing down like you would in a car). That has the effect of raising rpm so engine over-reving becomes and issue. You really need to get more exact numbers and think more about the expectations and by all means verify the boat is not overweight due to being water soaked. You need accurate WOT rpm, current prop numbers (diameter and pitch), speed and time to plane. You need speed measured by GPS and rpm by a tach that you absolutely know is accurate. Without that "firm" data you are only guessing.

Lastly, you really need to verify that the engine is healthy and capable of delivering full power. Have you tuned it up, verified good compression, throttle opening fully, timing set right, and if this is really a "worked over 305" what exactly was done to it to give it that title. Someone may have done an automotive hop-up job on a marine engine. The two don't necessarily translate. While the HP number may be higher, where on the torque curve that HP is developed is what makes a good marine engine.

 

[Scott Danforth]

Thank you for your help this far! It's quite possible the boat is water logged. It has some soft spots in the floor, or just one really. However, the 35 at 4500 was with the 4 blade prop. I guess I haven't measured with the 3 blade. Guess I need to find that old prop if I can and get the numbers off my current.

Fix your hull issues. Soft spots in the floor are a serious issue.

 

[MCNPathfinder]

Very good recommendations, and I thank you for all of them. I think we got the numbers off of there.

48-78118-17

Some numbers on the side say 17P, so I'm assuming that's my pitch? I was measuring over the weekend and it looks like WOT was 4500RPM. Between 35-40MPH depending on the waves and wind. I'm fine with losing some top speed if it means I get some more torque.

I'm aware of the soft floor thing too. The transom seems solid as do a few of the other structural components. It's definitely on my radar. I'm thinking very very soon it'll be dry dock time and you'll see me asking all sorts of questions on the restore forums.

I really appreciate your help!