3000SCR Difficult to Plane

[Innamorata]

When I say difficult I mean all conditions must be met. I have twin carbed 4.3s with alphas, and a gen set. That makes for an underpowered heavy boat. With a clean bottom, full fuel, full water, gear and three passengers, she will get up if you know how to dance with her, as in throttle, trim tab, everybody lean forward etc etc. then she will run full out 26-28 knots and cruise at 20-22.

As the bottom collects marine growth, you have to make adjustments, less water, less fuel, less people. It can be done with full weight but the dance is much harder. Obviously the answer is scrub the bottom more, but my question is; has anybody fitted a 4 blade prop to combat this? My fear is that I will lose cruising speed and it may over stress the old Alphas.

Ideas, thanks.

[mmwjr]

First what is the WOT for the boat?

Props in good shape?

Adding blade area (4 blades) increases load on the engines so pitch will need to be reduced.

Go to mercs prop calculator and see what it suggests.

Have you assessed the health of the engines? Compression, timing advance working,...

[Calguy]

I have a 1995 2400 SCR Bravo 2 Mercruiser 350 Horizon power. It is kept on an Air Dock boat lift so the bottom and drive are always clean. I cruise 30 mph at 3500 rpm and top speed of 46 at 5100 rpm. Get a boat lift and everything will change. Get those drives out of the water asap.

[Phillbo]

Apples and Oranges Calguy.

OP, call these guys. They worked wonders for a buddy that had a pig of a 2800.

Hill Marine - Stainless Steel Boat Props & Accessories


It's worth a call.

[Calguy]

Phillbo...... the point is.. none of these boats with 1 or 2 stern drives were never meant to be kept in the water full time....especially salt water. You want this guy to spend over 2K for new props?? It is a short term fix. I am still using my original 1995 Mercruiser props because the boat has been kept out of the water for since new. The top performance of of every boat depends on the cleanest hull and outdrives........

[Phillbo]

Sounds to me like he has an issue that can be addressed even with a clean hull. I was just passing on advice.

[Innamorata]

Thank you all for the great responses. Although my boat is almost 20 years old (I'm the second owner) it has very low hours. My engines are healthy as in they run very smooth and don't smoke, I spent quite a lot last year on new risers and manifolds due to inactivity from the previous owner. I don't think I'm getting all the WOT I should be, both engines seem to top out at a little over 4.5 k rpm, I haven't been out to check yet this spring. Last summer with a fresh bottom, she pulled 29 knots (gps registered) with just the Bimini top and low fuel load. I normal cruise at 22-24 knots , my memory is a little foggy.
I suspect the 4.3 V6 is just not powerful enough with the weight of a gen set, water heater, full fuel and water tanks to pick up a heavy boat like the SCR.

I would love to be able to pick her up, but I don't have a trailer or a truck that could pull it if I did. I keep her in a U.S. Navy MWR Marina on the Potomac River. That means I cannot modify the docks at all, so no floats or cradles. The best I can do is put her on the hard for winter. During the summer, I anchor at a sandy spot in about 4 feet and scrub the growth by hand.

My props are the stock aluminum but I had them rebuilt last summer.

[mmwjr]

WOT should be around 4600 so you are very close, dropping one pitch will increase it by around 250 rpm.

I think you what you have for this size boat with that power package.

[rodrigo]

Check for water in your bilge, you might be carrying some extra weight there.

[Innamorata]

The bilge in the engine room is dry but I suspect there may be some forward, is there an inspection plate in the cabin?

[shrew]

1) Where is everyone seated when you are getting up on plane? Out back along the transom, or up at the helm with you?

2) Have you tried it with 1/2 fuel and water?

3) Where are your coolers located? Out back or down below in the cabin?

Distribution of weight is going to be key. Amount of fuel, water and gear will as well. I stopped running around with a full tank of fuel along time ago. I only top off the tank when going long distances. The same with water. If you're going out for the day, then there is no need to carry more than you need.

Determing RPM's at Wide open Throttle (WOT) and determining if you are properly propped is the next step. Many folks over prop these cruisers to reduce RPM at cruising speed. The thought is they are reducing fuel consumption. This is a very, very bad idea.

[rodrigo]

hi Innamorata,
there has to be an access hatch midcabin or forward, on my boat I vacuum at least three body weights of water by the end of the season. make sure your trim tabs are operational, use them to help yourself to get up on plane.

[Innamorata]

I'm working on the boat all day tomorrow, I'll make every effort to dry the bilges and as far as weight distro :

I don't carry coolers, my passenger load is usually three counting myself ( 2 midship and 1 stern) no extra gear to speak of, and trim tabs operate normal.
I will reduce the water carried as well as fuel on a regular basis. I'm sure that will help. Im very interested in making sure I've got the correct props, I'm hoping that a tweaked set will give me the extra bit im looking for, I don't mind a top speed of 27-29 with a cruising speed of around 21-22 knots.

Thanks for all the good info and I'll repost after I spend Thursday cleaning and pumping out.

[shrew]

Try moving everyone up to the helm before getting on plane. Reduce fuel and water. That will help a bunch. I had a terrible time until I lowered the pitch. 24P on a Bravo III with a single 454 and later 496 was way too much. Now at 22.5 it planes and runs WAY better.

[Innamorata]

All great ideas, I appreciate the help, that's what I asked for and now I've got something to work with. Just as an after thought; I know my boat was originally equipped and ordered for value what with the 4.3s and Alpha I drives, but I noticed they both turn the same direction. I'm wondering if the non counter rotating props create too much side action. I have no idea if that's even a thing. I never noticed any adverse effects, even in reverse, but then again I don't have a baseline to compare it to.

[mmwjr]

Other than when docking I doubt not having counter rotating props will make any difference

[Innamorata]

I seem to have a great starting point to tweaking my situation for the better, thank you all.

Yesterday I went out with a full fuel load, almost no water, full camper enclosure (so no aerodynamics at all ) and only 300 lbs in passengers. I was able to jump up and pull 28 knots indicated via GPS. I'm sure with canvas down she will hit 30 knots. The only shortcoming was the WOT RPMs , starboard would max out at 4600rpm while the port would pull 5200rpm, I could throttle back and balance at 3800-4000 for a nice 23 knots. I suspect the linkage might be wanting or maybe a auto choke not flowing as much air.

Lastly, at speed, she lists to the port about 10 degrees, at under 10 knots she does not list that I can notice. It may be a trim issue (duh) as both my outdrive trim gauges are inoperative, I just put both down all the way and they look the same. Any ideas?

[mmwjr]

Port engine is turning to high, have you verified both props are the same diameter and pitch?

Regarding the list it seems to be either drive trim or trim tabs being offset.
Since off plane no list the boat sounds like weight distribution is good.

[Innamorata]

Thanks, I suspect that anything over 5k is too many rpm, I had both props resurfaced last fall, they were definitely the same size, I didn't check pitch although they looked the same.

Both my props turn the same direction, I've read a bit about prop torque causing a port list at speed. I do plan on exploring 4 blade props, I could give up a couple knots in exchange for more bite.

[mmwjr]

Yes both props turning the same direction can result in some list, have you tried to use the trim tabs or drive trim to compensate?