New Prop not getting full WOT RPM

[bman1bpm]

I bought a new prop this season to help get my boat on plane faster. My boat is a 2001 bayliner 23' cuddy with a 5.0 Mecruiser (carburetor). My original prop was a 15x17" 3 blade, I got 4400 RPM with it. The WOT RPM range on the motor cover is 4400-4800 rpm

I bought a Quicksilver Nemesis 48-8M0055535 14" x 19" pitch Standard Rotation 4 Blade Aluminum Boat Propeller based on the tool on Mercury's site. I wanted to be able to get on plane faster.

With just me on the boat the max WOT is now 4000. It Seemed to rev more freely than with the 3 blade prop and it did get on plane faster, but I wasn't getting full RPM. We went out with a fully loaded boat yesterday, 6 people with gear for the day. I couldn't get the boat to plane at all, my max RPM was 3000.

So clearly I need to go down in pitch. My question is how much? If I use the 1" = 200 rpm figure then going to a 14" x 15" will give me 4800 rpm with just me on board, but does that mean I still will only get 3800 with a full load?

 

[alldodge]

original prop was a 15x17" 3 blade

Going from a 3-blade to a 4-blade you need to drop to pitches. Had 17 an need to go to a 15. Going to a 4-blade 19 is like going to a 21 3-blade

 

[bman1bpm]

They don't list a 15 pitch but there's a 14. Will the 14 give me better acceleration than the 15? It may put me over the 4800 WOT limit with a light load but I can watch the tach and not give it too much gas right?

 

[jimmbo]

A 23ft boat is asking a lot of a 5 liter engine. With the original 17" you were at the bottom of the WOT range, which is 4400 - 4800 rpm. A 3 blade 15 would have placed you at about 4800. Your 4 blade overloaded engine even when it was just you in the boat(4000 rpm), adding 5 people and gear, brought it too its knees(hope you didn't melt a piston). A 14" 4 blade will probably place you near 4600 - 4700 rpm. But you will have lost several mph compared to a the original 17 or a 3 blade 15

Running too little pitch and figuring you can just hold back the throttle really isn't that great of an idea. The fuel delivery curves for the carb are based on certain expected airflow and vacuum at any given throttle setting and rpm. If the engine is over revving due to wrong prop, the engine when throttled back, can produce higher than expected manifold vacuum, which might be high enough to close the power enrichment circuit. this may result in a mixture too lean for speed/load the engine is really seeing.
Your combination of boat-engine will probably require you to change props, depending on the load, more often than you like.

 

[bman1bpm]

Oh she's definitely lacking in the power department. My 3 blade prop would top out at 32 mph at 4400. The 4 blade gave me 28 add 4000.

The reason I went from 3 to 4 blades was to try and get on plane at lower rpm. I found myself having to go nearly full throttle just to get on plane. Do you think I'd be better off with a 3 blade 15" Over a 4 blade 14"? Or is it just hard to say without buying more props and driving it ? My boat also doesn't have trim tabs and I don't think that's helping my situation

 

[alldodge]

While motor rpm is a factor to fuel consumption, load has a larger factor. Reving up the motor in neutral burns hardly any gas, under load burns much more. So running the motor on plane at 3800 with the best match prop, is better then running it at 3400 with the wrong one.

IMO with the load on the boat you need more motor or less load in the boat, but without going that route I would suggest the 15 3-blade with the largest diameter you can find. The more blades in the water, the more drag on the motor

 

[dingbat]

I found myself having to go nearly full throttle just to get on plane.

never owned or operated a boat you could “feather” on to plane. Need that burst of HP to get over the hump of water your pushing getting up on plane.

Go to a 15x15 and bury the throttle until on plane then back off to your speed.

 

[bman1bpm]

never owned or operated a boat you could “feather” on to plane. Need that burst of HP to get over the hump of water your pushing getting up on plane.

Go to a 15x15 and bury the throttle until on plane then back off to your speed.

I should rephrase, I had to go nearly full throttle to stay on plane

 

[Maclin]

Just curious, what altitude do you boat at?

 

[bman1bpm]

I'm at sea level, Chesapeake Bay

 

[alldodge]

I should rephrase, I had to go nearly full throttle to stay on plane

Most likely even with the change in prop, you do not have enough motor with the boat and load you have. One of the sayings 'you cant get there from here"

The attitude is if your bow down, up, or some other, not elevation

 

[bman1bpm]

Most likely even with the change in prop, you do not have enough motor with the boat and load you have. One of the sayings 'you cant get there from here"

The attitude is if your bow down, up, or some other, not elevation

With my 17p 3 blade I was at least able to get on plane, with a full load but it did take a bit of time. Do you think trim tabs would do me any good or am I still going to be fighting a losing battle with the 5.0 motor?

 

[jimmbo]

What condition is the engine in? When was the last time a compression test was done? and what were the numbers? Is it na good state of tune?
A 5 liter can push the boat to a respectable speed, when propped properly. It will require prop changes for heavier loads and water sports. Myself, I would want a 7.4 in a boat that size

 

[bman1bpm]

The engine runs great with 4 people on board, the 6 people seemed to cause the problem. I haven't done a compression test but I don't think I've got any engine problems. With the 3 blade prop I could take 6 people no problem, just took some time to get on plane, that's why I changed the prop to the 4 blade. I was hoping to get on plane and faster and be able to stay on plane at lower speeds so I can ride more smoothly in rough water. The 4 blade prop does help with that when I've got 2-4 people on board, but with 6 I couldn't get out of my own way. Would it be worth ordering a 4 blade 17p or even 14p (they don't make a 15). Or should I just abandon the whole 4 blade prop idea, go back to my 3 blade and get some trim tabs?

The boat had an optional 7.4L but I'm not lucky enough to have that model, I'm stuck with the base 5.0. Changing motors isn't in the budget for this boat.

 

[BCreech]

I have a Thompson Daytona 225 cuddy. It has a 5.0L , originally had a 15x15 3 blade prop. Maxed out at 32 mph and took forever to plane. I bought a Turning Point hustler 4 blade 15x15. Now I plane a lot quicker and max out at 36 mph. I also's keep my water tank full to keep weight on the bow.

 

[bman1bpm]

I have a Thompson Daytona 225 cuddy. It has a 5.0L , originally had a 15x15 3 blade prop. Maxed out at 32 mph and took forever to plane. I bought a Turning Point hustler 4 blade 15x15. Now I plane a lot quicker and max out at 36 mph. I also's keep my water tank full to keep weight on the bow.

That's good to hear! So I think mercury's prop finder gave me bad info because it suggested I go up in pitch when going from 3-4 blades. Maybe I'll give that turning point a shot. Merc doesn't make a 15x15 but l think that's the size I really need.

 

[jimmbo]

If you go with a Turning Point, inspect the prop before you buy it. The QC of that brand is lacking, I have unboxed many TP props and could sing, 'One of these Blades is not like the others, one of these Blades just doesn't belong...'

 

[Sea Rider]

What's the min-max wot rpm range factory stated for that engine ?

What was the wot rpm achieved with 6 up with previous 3 blade prop ?

With how many passengers do you usually go boating with ?

Happy Boating

 

[dingbat]

That's good to hear! So I think mercury's prop finder gave me bad info because it suggested I go up in pitch when going from 3-4 blades. Maybe I'll give that turning point a shot. Merc doesn't make a 15x15 but l think that's the size I really need.

Interesting handle.....fished out of BPM for years......

Don't put blinders on to a 3 blade prop.
My boat is the same length as yours only heavier. You have 20 HP advantage.

Ran a 14.25 x 17, 4 blade up until this spring. Liked the hole shoot, but my slip numbers where crazy at the higher RPM.
Swapped out the 4 blade for a Merc 15 x 15, 3 blade this Spring.

The results....got my WOT up to where is was supposed to be providing a noticeably better hole shot. Increased my top end from 36 mph to 40 mph while reducing my fuel burn by roughly 10% (fuel monitoring system).

Don't miss the old 4 blade one bit.

 

[bman1bpm]

I went ahead and ordered a turning point. Wow their prop selector tool was very comprehensive and no comparison to the Mercury one. I highly recommend their website! It suggested a 15x15, I'm gonna give the 4 blade a shot again. I'll give it a good look over before I install it. They have a return program so if it doesn't work out they'll swap it out for a different prop for $25. Says it'll work with my quicksilver hub too so I'm not out too much cash on the new set up. I'll keep you all posted with my results in a week or so.