Correct prop size/pitch for 24ft deck boat

[arejaye]

I recently purchased a used 2007 Starcraft 24ft deck boat and I noticed the prop was not the OEM factory prop. I’m getting high RPM’s but not much top end speed. I made a mark on the prop and hub and it’s not a spun hub so I’m thinking I’m just under propped. Currently it has a 14.5x19 four blade prop.
Can anyone recommend what prop I should be using for this size boat and all around performance? It has a Volvo Penta 5.0.

 

[Starcraft5834]

what rpms are you getting? assuming your tac is correct.. thinking that motor should be in the 4800 range.. if your higher than 5000... increase pitch.. each step up in pitch will decrease rpms bout 200.. if you reduce your pitch..rpm will go up bout 200.. 24 foot boat.. would think 4 blade is wise as that's on the big side of things...

 

[arejaye]

Thank you for the reply. I was out Saturday and was running around 5500 and barely getting over 20-25mph. That was just my wife and I. What’s the best/easiest way to check to see if the tach is accurate?

 

[arejaye]

Also, if I increase the pitch, will it be a dog on takeoff to get on plane?

 

[Ming15237]

Thank you for the reply. I was out Saturday and was running around 5500 and barely getting over 20-25mph. That was just my wife and I. What’s the best/easiest way to check to see if the tach is accurate?

Something here really is NOT right, either your speedo is off BIG TIME, or your RPM gauge is.A 5.0 Mercruiser in that boat should reach fuel cutoff at right around 5k rpm's. My guess is your rpm gauge is way off. BTW that pitch 3 blade prop is about right for that size and weight boat. a 14.5 4 blade seems like way to much prop for that boat. I have nearly the identical boat with a 5.0 MPI Mercruiser, and I run a 14.5 X17 4 blade for water sports and the boat will still bump 50mph.

 

[arejaye]

The current prop is a 4 blade 14.5x19. I agree I believe the tach is definitely not right because it didn’t sound like I was over revving. But the tach is what I always watch on my boats do it was scaring me to see it go up that high.
I might also add that it was drinking A LOT of fuel so I’m not sure if that says anything either?

 

[arejaye]

My apologies, I’m running a 14.5x17 not 19.

 

[ahicks]

Sounds like you have a ski prop on it - set up/sized to get the tuber/skier on top of the water as quickly as possible. If that's not you main priority, a 19" may work better.

If it were my boat, I'd be looking at 4 blade designs.

 

[Scott Danforth]

first, verify your speed. download a free GPS app for your phone. you need GPS speed

second, verify your tach. either get a good shop tach and connect it to the coil and read the RPM, or get a photo tach and read the engine damper.

after you verify all that, then we will discuss the prop.

because at 5500 RPM for any length of time, your pistons would have swapped bores and you would be rebuilding a stock 5.0 either your tach is reading 133% (set to 6 cylinders and not 8 cylinders) or you are going much much faster than you think or you have a serious prop issue

 

[Starcraft5834]

As Scott said, would'nt think your engine survive 5500rpms for long.... 4 blade is smart.. what pitch is a guess until you verify your tac.

 

[arejaye]

first, verify your speed. download a free GPS app for your phone. you need GPS speed

second, verify your tach. either get a good shop tach and connect it to the coil and read the RPM, or get a photo tach and read the engine damper.

after you verify all that, then we will discuss the prop.

because at 5500 RPM for any length of time, your pistons would have swapped bores and you would be rebuilding a stock 5.0 either your tach is reading 133% (set to 6 cylinders and not 8 cylinders) or you are going much much faster than you think or you have a serious prop issue

how would I check if the tach is set at 6 cyl instead of 8cyl? I will gather the other info needed tomorrow. Thank you

 

[Scott Danforth]

how would I check if the tach is set at 6 cyl instead of 8cyl? I will gather the other info needed tomorrow. Thank you

Look under your dash at the switches on the tach

 

[arejaye]

Couple updates here..
1. The tach was on the 6 cyl setting, I changed it to 8 cyl so that is fixed.
2. It was running rough at idle and barely got on plane so I knew this was not prop related issue. Mechanic came and did full tune up. Problem ended up being clogged injectors. Boat runs like a top now.

However, I still think I’m not propped right for my needs. I rarely pull tubes, typical use is 4-6 adults.

Now that the boat is running right, at WOT I’m at 4800 rpms and top speed of 41 mph. This is running a 4 blade 14.5 x 17 prop. The mechanic stated he was almost positive it didn’t ship from factory with a 4 blade, and that it was probably changed out by the previous owner that used the boat for pulling tubes and skiers.

So it’s back to square one. What is the OEM prop that comes on these boats? I believe it is a 3 blade. If I go with a 3 blade, what would be a good size if I want a little more top end, but still be able to get on plane pretty quick? Also, my gas mileage is not that great with this 4 blade prop...will a 3 blade prop provide any difference in gas mileage?

TIA again for all of the input

 

[Scott06]

Couple updates here..

However, I still think I’m not propped right for my needs. I rarely pull tubes, typical use is 4-6 adults.

Now that the boat is running right, at WOT I’m at 4800 rpms and top speed of 41 mph. This is running a 4 blade 14.5 x 17 prop. The mechanic stated he was almost positive it didn’t ship from factory with a 4 blade, and that it was probably changed out by the previous owner that used the boat for pulling tubes and skiers.

So it’s back to square one. What is the OEM prop that comes on these boats? I believe it is a 3 blade. If I go with a 3 blade, what would be a good size if I want a little more top end, but still be able to get on plane pretty quick? Also, my gas mileage is not that great with this 4 blade prop...will a 3 blade prop provide any difference in gas mileage?

TIA again for all of the input

I would contact Starcraft to determine what was originally shipped with the boat or look in their literature. Generally boats are shipped with fairly generic props that would suit a wide range of uses not necessarily how you want to use your boat.

A couple of thoughts on your current set up. Since you are getting 4800 rpms I'd think you are propped correctly. Your typical use of rarely pulling tubes but having a fair amount of weight in the boat (4-6) people given the engine and size of the boat I suspect the previous owner put this prop on to improve performance over the three blade.

Does your boat get on plane super quick ? As you are looking for both more top speed while still getting on plane quick, with swapping props you only get one or the other, increasing pitch will increase speed if the the engine has enough power to swing the pitch, and you will lose planning speed. The only way to get both more top speed and holeshot is to add a bigger engine.

If you want to try a 3 blade Id see if you can get a prop shop to try out a 19" pitch, hopefully you can try before you buy or get an aluminum like a michigan vortex or black max untill you hone in on what you works.

Fuel economy is related to how far you push the throttle doubt you'll see much difference

 

[ahicks]

You can spend a lot of money looking for that last couple mph if you wish, but we are talking about a deck boat here. There's nothing available that's going to get you more than maybe another 5 mph tops, and that prop is not going to get the boat on plane as quickly as the prop you have now.

Further, the 4 blade props will hold the boat on plane at lower speeds. Nothing dramatic, but generally pretty noticeable. If you're interested in gas mileage, this feature (holding the boat on plane at lower speeds) should be of primary interest, as it this speed where you can make max mileage.

Bottom line, 4 blade props are generally considered very desirable when talking about deck boats. That's not to say you MUST conform though. "Different strokes" concept certainly plays well with boats.....

 

[Scott Danforth]

glad you have the tach and motor sorted now

you need more motor to get more top end, or less boat.

if you want gas mileage on a boat, switch to a sail boat. your motor will burn 0.4# of fuel per HP per hr. no, a properly sized 3-blade wont save you any fuel. unlike a car that literally only takes about 5-10hp to move down the highway at 65mph, your boat will take all 250hp that motor has to move your boat at 41mph with the throttle pegged.