3.0L Alpha 1 Prop Selection

[taustin52]

Hey guys, I need a little advice choosing a new prop for the family boat. I have a 1989 Larson bowrider runabout, 17/17.5 footer. It is equipped with the mercruiser alpha one drive, 3.0L inline 4 motor pumping out 135/140HP. Checking the owners manual the top RPM is 4600 RPM's, I unfortunately don't know what it actually runs, but I believe its around 4500 with the current prop. With the current prop she will do like 32mph, which to me seems kind of low. The current prop is an aluminum prop i believe made by quicksilver. The model number on the side reads 78120 AA0L 19P I believe its kind of hard to read. I'm guessing the 19P means a 19 pitch, but i'm unsure of the diameter. The reason we are looking for a new prop is the current one is really chewed up. We had it refinished and the marina said they took off like 3/8 of inch in order to refinish it, and I believe the boat suffers in performance from it. We use it mainly for tubing and skiing, so i'm looking for a prop thats going to give me a good hole shot while maintaining a descent top end (if thats possible). Any recommendations or advice would be greatly appreciated, and please let me know if you guys need anymore information

 

[Maclin]

Re: 3.0L Alpha 1 Prop Selection

A boat with a correct prop is a wonderful thing, worth spending time (and unfortunately $) to get it right.

To do good prop calcs you really need to know 1.) Drive ratio 2.) ACCURATE rpm at full throttle 3.) ACCURATE mph at full throttle

Your ratio is probably 2.0, and with a 19p prop at 4500 rpm with 12% slip the speed should be 35mph. All of this assumes a prop in good shape and of course accurate numbers.

The 19p is a good all around ratio. But if it is slipping in the water due to condition of the blades then you will lose grip, and loss of hole shot and speed are the results.

Since you can hit 4500 rpm now I think the 19p is a good start, meaning get a good prop on there with 19p and see how that works for you. Be aware that the good prop may drop your rpms some with the extra bite. But I would start there. You can maybe find a prop shop or Dealer that will allow you to swap around if they know you are buying one from them.

I had a bad set of props that I used for a while. The boat could hit top RPMs but the speed was not quite right, way too low, and hole shot was just ok. I looked up what speed I wanted on a chart from my drive manufacturer and decided to go down 2 steps in pitch (from 25 to 23 equivalent) and gained 4 mph, this just due to that bad bite the older props had. Later I fixed the bad set and tried them and gained 3-4 more MPH but it struggled a little out of the hole. I stayed with the lower pitch set because I mainly do towing watersports and the lake I frequented was small, not a lot of 50+mph cruising room. These hole shot like crazy and can still hit 46+mph and optimum cruise is at 40-42mph.

 

[90stingray]

Re: 3.0L Alpha 1 Prop Selection

If they removed that much material then the prop will not have as much bite into the water... thus the slow mph. But with a damaged/ modified prop you are only turning 4500... i would think a full sized 19p might be too much for you. You may find that if you drop to a 17p, you might actually gain mph... since it puts the motor closer to max hp mark. Just my $.02

 

[steelespike]

Re: 3.0L Alpha 1 Prop Selection

From my limited experience typically a 17' -18' 3.0 will hover around 40 mph with a 19-21 prop.
Lightly loaded and in excellent tune.Likely wouldn't do well in water sports.
If you run with a big crew and do water sports a 17 will work well but you may have to watch the rpms lightly loaded at wot.If youve been reasonably happy with the 19 stick with it.

 

[taustin52]

Re: 3.0L Alpha 1 Prop Selection

Thank you everyone for the input, I'm kind of torn right now whether to do the 17 or the 19. I'm not sure how I feel about the 19 because the one we are running is in horrible condition and missing quite a bit of material which I'm sure is affecting performance. That said I think I'm leaning towards the 17 because we are mainly going to use it for watersports. Should I be looking for a certain diameter to go with the 17/19 pitch? I'm a little worried about the rpm, we are taking the boat out this weekend so I will find out exactly what the wot rpm is and report back. I feel like you guys could give me a better recommendation with that information.

 

[fishrdan]

Re: 3.0L Alpha 1 Prop Selection

I feel like you guys could give me a better recommendation with that information.

With accurate numbers (RPM, MPH) while running a good prop they probably could. But, not know the exact RPM, MPH and running a questionable prop, it's hard to say. Boat's vary too much and what might work great for one hull might not work well for another. Throw in that your boat could have water logged foam, or lot of extra gear it's hard to say what would be a good choice.

On my heavily loaded 18' deep V (around 2500# + people) my 140HP Mercruiser is running 34MPH at 4300RPM with a 15.5x19P prop. I'm guessing I could pick up the extra 300RPM if I swapped to a 14-14.5 diameter 19P prop. If I was into watersports, I'd drop to a 17P to get better hole shot. I'm actually thinking of getting an 18P 4 blade since my boat is stern heavy and it takes a while to plane with several people on board.

A couple of suggestions: Buy (borrow) a decent used 19P prop and get some accurate numbers, then buy a new prop. Go to a local prop shop that will let you demo a couple props, then buy the right one, it's going to cost more. Buy a 17P from Iboats for about $100 and hope it's the right one.

For diameter, it's more about prop design and not a big consideration if comparing a 14 and a 14.5" prop, even up to 15" D for a 17" P prop. As pitch goes down, diameter goes up, 14x21, 14.5x19, 15x17, etc. Large diameter props 15.5 - 16" are heavy load props and not what you are looking for. I bought my 15.5x19P prop before I knew better, smoking deal from Iboats years ago.... The prop runs OK and will keep the boat on plane at low speeds, but I know there are better ones available.

 

[taustin52]

Re: 3.0L Alpha 1 Prop Selection

Thanks for the info, I'll have to see if i can find a shop that will let me try out a couple different props. From what I'm reading I think a 15" 17P prop is what will work best, but like you guys are saying I'll just have to try it. Another question though, I was looking at the props here on Iboats, and I noticed that the props also come with a new hub assembly. Do all props come with a new hub assembly or is it recommended that I replace the hub when i replace the prop? Also would I benefit from a 4 blade prop? More surface area = better hole shot?

 

[90stingray]

Re: 3.0L Alpha 1 Prop Selection

Yes the 4 blade will help holeshot. I am running a 17P 4 blade Solas from iboats. They are pretty darn cheap w/o the hub kit! If you already have a prop that uses the hub kit... then you are ok. You will have to tap out the biggest piece from the inside of the prop and swap to the new one. But if you have an older style that is pressed in... then there is no swapping to the new prop.

 

[fishrdan]

Re: 3.0L Alpha 1 Prop Selection

Some props come with the hub built-in, others use a replaceable hub. A replaceable hub can get wedged into the prop pretty tight so I would get a new hub with a new prop, so you can leave the hub in each prop. For the little 3.0, I don't think the replaceable hub is a big deal as the engine doesn't have tons of torque that could spin the hub, still possible I guess... Also, all props don't use the same replaceable hub style, Hustler uses a different hub than Mercruiser (Quicksilver), though some are interchangeable between prop brands Quicksilver/Michigan Wheel. Clear as mud, eh'?

On the 4 blade prop, they are supposed to work well for watersports because of increased holeshot, though they will reduce top end speed a bit. From what I gather, if going to a 4 blade prop you should reduce pitch by 1#, 18P 4 blade would replace a 19P 3blade.

 

[Scott Farley]

Re: 3.0L Alpha 1 Prop Selection

I have a similar setup. I replaced my quiksilver 19" aluminum prop. with a Turbo brand 19" used stainless prop I bought on ebay. I am amazed at the all round performance gain with the stainless prop. Much faster to plane, and about 4mph better top speed. The quiksilver was new this spring, so they are in similar physicall condition. Looking at them, the only difference I see is the stainless prop's blades are much thinner than the aluminum one. I am very pleased with the turbo prop. I haven't pulled the tube with it yet, but my boat could only reach 3400rpm with two adults on a three person tube. I hope this doesn't muddy up the waters any!