prop question on deck boat

[vip215dl]

I have a 21.5' deck boat with a 19x15 pitch prop. Boat is running around 45 mph at 3700 rpm then acts like it hits rev limiter. I have to back off the throttle a little bit to keep it off the rev limiter. Should this boat rev to 4500 or more? It's a 5.7l mag mpi year is 1998. I just figured this boat ought to run in the mid 50's. I also had 8 people in the boat if that matters or not..

 

[steelespike]

Re: prop question on deck boat

Someone with more experience with your setup should be along.
It will help if you could be more specific about your boat and motor I would think a 5.7 would beat 50 easily.
I think you mean you have a 15 X 19 prop (Diameter X pitch)
I can't be too specific about your setup except to say I think your tach is off
(low) I don't know your rated rpm but would guess in the 4,600/4800 range.It may be on the motor.Also your gear ratio should be on the outdrive.
A 19" prop seems really small to me probably for skiing with a load aboard.Probably would run about a 23" prop for cruising and speed.Don't take that to the bank.I'm sure someone can pin it down better.

 

[vip215dl]

Re: prop question on deck boat

i'll go out and see if i can find the gear ratio.

 

[vip215dl]

Re: prop question on deck boat

ok, i told you guys the wrong prop number, it is a 15x17 and the number on the lower unit is a 1.50r i guess the r stands for ratio. boat weighs around 3800 lbs i think. This prop will get the boat up on plain around 18 mph which is pretty good for pulling a tube. let me know what you guys think...

 

[steelespike]

Re: prop question on deck boat

Maybe I'm crazy but that prop sounds to me like the guy didn't want to give up his good prop.so he stuck any prop on there. We see 17" pitch in 19ft 3.0 bow riders,
for sking, with the caution to watch for over reving when lightly loaded.
I think you can see that if your motor would be at about 4,800 with a 23" prop that your 17 will reach somethimng like 6,000(very bad) if allowed to.
If it works for you just watch the rpm and get your tach checked.

 

[vip215dl]

Re: prop question on deck boat

so i wonder since it's only a 17, that it's not putting enough load on the engine, that it protects it's self by only allowing it to go to 3800, kind of like reving it in neutral, which i haven't done.

 

[steelespike]

Re: prop question on deck boat

No, if there is a rev limiter it would kick in slightly higher than the max rated rpm'
its not related to the tach.
At the very least you need to verify your tach before you do anything else.

 

[vip215dl]

Re: prop question on deck boat

I'm testing the tach later today. So if i go to a 21p or so, would i go with a 3-blade or a 4-blade, to help it out of the hole for tubbing..

 

[vip215dl]

Re: prop question on deck boat

anybody?

 

[msojjo]

Re: prop question on deck boat

I was having the same problem you were this spring. I have a 350 mag in my 20 foot boat, and you would think with all that horse power it would just shoot you out of the hole. My boat is 3300 lbs, dry, so add 50 gal of fuel and people i'm well over 4000lbs. I think i saw you are at a pretty high weight also, anyway, i have a 17 pitch prop, but i do alot of sking and tubing, i just watch wot. the 350 mag recomended rpm is between 4800 and 5000, with the limiter hitting around 5250. I would first check your tach, then if your not gaining your full rpm full tune up, compression check ect.

 

[vip215dl]

Re: prop question on deck boat

the tach was off, they said around 600 rpm. so now i'm looking at 4000 lbs, a 17p prop running 48 mph at 4400 rpm. So now what i'm wanting to know is if a 19p 3 blade still get me up out of the hole at a lower rpm for tubing and help a little bit on the top end? or would a 4 blade be better.

 

[steelespike]

Re: prop question on deck boat

I think your 17" prop is way under sized.You have other issues.A 17" pitch should leap out of the hole. And over rev easily if allowed.Is the bottom clean and smooth?
Motor in excellent tune?Is it running on all cylinders.Throttle plates opening all the way?A compression test may be a good idea.

 

[vip215dl]

Re: prop question on deck boat

mabey i'm not saying something right. The boat pops up on plain easy with a 17p and over revs easily. What i'm wanting is a prop that will still plain at a fairly low speed, and has a better top end without hitting the rev limiter. Engine runs perfect, nothing wrong with it. boat runs perfect!!!

 

[steelespike]

Re: prop question on deck boat

Well its confusing you say your running at 4,400 on a 17" prop.
Then you must be throttling back to maintain 4,400 thats not clear usually when
someone quotes rpm and speed it at wot other wise it doesn't mean much toward selecting a prop.It is best to try to select a prop you know should reach your max rating.That said a 19 will lower rpm 300 to 400. a 19" 4 blade should lower it a little more and improve hole shot over the stock 3 blade 19.You may still have to watch the over rev.

 

[vip215dl]

Re: prop question on deck boat

I'm going to check everything out tomorrow. I also ended up with a 14.75x21 3 blade that i'll try out also. If i don't like it, then i'll probably go with a 4blade 19p.. Oh yea and i'll check the tach with both props to see if the boat place lied to me about the tach being off 600 rpm.. But like i say, the engine runs great, just seams to hit the rev. limiter to early..