Not the usual RPM question
 

Hi guys!

Back again after the next run of our Davanti. We had 5 adults and 3 kids on the boat today. What I ran into was interesting. The boat ran fine as in launching and engine temperature, etc. But here is my dilema.

On the driveway when warming up. and even after we came back, the engine was able to rev up to 4000+ RPMs. But in the water, how much ever I tried, the max it would go was 3000RPM. The speedometer would not even move on the needle. I was crusing fine but I could not take of or get on plane.

Not sure if I am explaining it right, but simplistically, the boat did not seem to have any power to pull or get on plane. What should I be looking for?

Thanks,

Sanu

Re: Not the usual RPM question
 

here's a list of possible causes....

1. Fuel condition. Type & Octane or possibly old fuel
2. Fuel vent clogged?
3. Propeller pitch or diameter, damaged blades
4. Restricted fuel pickup tube or anti siphon valve
5. Crankcase oil volume, high oil level can cause aerated oil and lifter collapse
6. Restricted carburetor air intake (clogged flame arrestor)
7. Restricted exhaust system (broken exhaust shutters/flappers) in engine transom shield or drive
8. Poor cylinder compression
9. Carburetor defective, or wrong type.
10. Fuel pump pressure and vacuum
11. Boat overloaded, improperly loaded, or improperly trimmed.
12. Engine Overheating
13. Engine timing and ignition system operation
14. Remote control cables and linkage for proper travel to open throttle plates fully.

Re: Not the usual RPM question
 

Mikey,

Thanks for that response. I thought of a most of it but there are a few confusing things about it for me:
1. Fuel condition. Type & Octane or possibly old fuel
SKC - On the driveway, the engine rev-ed fine I was able to rev it up to 4500 RPM without even breaking a sweat. So I am not sure of the fuel condition.
2. Fuel vent clogged?
SKC - Same question as above
3. Propeller pitch or diameter, damaged blades
SKC - The prop looked fine when I looked at it after pulling the boat out of the water
4. Restricted fuel pickup tube or anti siphon valve
SKC - Would this not cause it to not rev up at any time?
5. Crankcase oil volume, high oil level can cause aerated oil and lifter collapse
SKC- the volumes looked ok on inspection
6. Restricted carburetor air intake (clogged flame arrestor)
SKC - How do I check for this? Would this only have an issue under load?
7. Restricted exhaust system (broken exhaust shutters/flappers) in engine transom shield or drive
SKC - How do I check for that?
8. Poor cylinder compression
SKC - Had it checked last week and it was 175/180 all around
9. Carburetor defective, or wrong type.
SKC - How come it revs fine out of the water?
10. Fuel pump pressure and vacuum
SKC - Again, why only in water and not on land?
11. Boat overloaded, improperly loaded, or improperly trimmed.
SKC - Possible. Since we had 5 adults and 3 kids. And was my initial thought as well. Although I had managed somehow, not sure how to get it on plan at around 3800 RPM after trimming/tilting at different angles.
12. Engine Overheating
SKC - No. The engine temp remained at or under 175 throughout. I was very conscious of the waterpump/impeller issue we had a couple of weeks ago.
13. Engine timing and ignition system operation
SKC - how do I check for that?
14. Remote control cables and linkage for proper travel to open throttle plates fully.
SKC - How do I check for that?

I hope I dont come across as a smartass asking the same question with regard to how come it worked fine on the driveway with muffs, but not under load in the water?

Sanu

Re: Not the usual RPM question
 

I'm going to go with the over simple approach - in the driveway = zero load. In the water, factor in the boats weight/bouyancy, plus passenger(s), weight, and fuel (# gallons x about 7lbs I think).. A simple test to eliminate this would be to run the boat in similar conditions with just you on board and see if there is a difference. If not, then start looking elsewhere.

Re: Not the usual RPM question
 

All of the reasons that MikeyT posted are possibles... I would look at prop pitch first, as it would be easy to identify. The pitch is listed on the side of the prop. If your pitch is too high its like trying to take off in fifth gear in your car. Thats what was wrong with my 238 when I bought it. Moving an adult or 2 into the front of the boat will help it plane out, but doesn't solve the problem. Another thing that you might want to look at, is make sure your outdrive is trimming all the way down as far as it's supposed to go. If it's sticking up some, then it will cause the boat not to plane. Hope this helps, Interested to hear the final verdict.

Re: Not the usual RPM question
 

I am sorry that it took me this long to put the boat in the water after my last post, but I am glad to say that it seems like the problem was with the weight in the boat with 5 adults and 3 kids. This weekend we ran her with 3 adults and 2 kids (just our family and no guests) and the boat got on plane at around 24mph (on the speedometer). WOT was 38 mph at 3800rpm. She was amazing. The first weekend of boating without any concern and a great experience.

I am guessing my speed at WOT was lower because of it being fully loaded with all of us. But what we found was that we were more comfortable at 24-25mph (planing speed).

A nice footnote though: we pulled into the dock and I was tying her up to the dock and I overheard one guy tell another - "Wow, that's a Mariah. The Cadillac of boats". What can I say, but that felt good.

Re: Not the usual RPM question
 

Sanu, If your WOT is only 3800 RPM's you need to drop your prop pitch. You're putting too much strain on your engine where you are now. You will see improvement on your planing as well.

Re: Not the usual RPM question
 

I am running a 14.5x19 now. What should I try?

Re: Not the usual RPM question
 

Each 1" drop in pitch increases RPM by about 200, so if you're really running at 3500 you should be running at about 4500, but I can't believe you're actually running a 19" and gettingn3500 RPM. CHECK it again, but my 222 with a 350 mag mpi came with a 19" prop and imwas turning about 4500'RPM

Re: Not the usual RPM question
 

I just bought a 1998 Jubilee with a 21 pitch prop. It did well empty, but when I loaded it up, it was lethargic. My 5.7L was maxing out at 3800 RPM, it was sluggish, the prop was cavitating ... A BIG MESS! I switched to a 15 pitch prop and what a difference! Fully loaded, that is 9 people and 50 gallons of fuel, it pulled strong, pulled a skier, pulled a raft and was maxing out at 4300 RPM and 35 MPH.

However, a couple of deck-boat owners told me that they are running 19 pitch props and fully loaded they are making 45 MPH at 4500 RPM. True or boasting? I'd appreciate candid advice.

Re: Not the usual RPM question
 

I verified it to make sure it was a 19 prop. Attached is a pic. I wonder if it was because it was fully loaded with 5 people with a full tank of gas. Attachment 3230

Re: Not the usual RPM question
 

I tested it again today with just my wife and I on the boat with a full tank of gas. WOT was 34mph @ 4000 RPM. Cruising was at 24mph @ 3000 RPM. Both speed measurements on the speedometer since I don't have a GPS yet.
Sanu

Re: Not the usual RPM question
 

My new to me boat, Davanti 225, 5.7 had a stainless 14x19 3 blade. ran poorly and had a hard time getting on plane. A couple of time she wouldn't. 3 people and a fat dog full fuel load. 4200 rpm max at 42 mph.

New plugs, one was much darker than the other 7, new cap and rotor. New 14x17 4 blade.
Whole different boat. Planes in a couple of seconds, reaches 4600 rpm at 45 mph.

Question, what is the red line of the motor, as she will continue to gain revs past 4600 rpm with the 17 inch prop?
Gonna try the 19 again, maybe buy an 18.

Re: Not the usual RPM question
 

Red line most gm engines is 5 grand stock the 496 I think is 5200 your wot should be between 4000-5000rpm 4600 would be a safe place to be going much over you are near the limit.