How should boat sit in the water?

[HighPlainDrifter]

I just got an old Aluminum boat. It came with 3cylinder 1979 Johnson 55HP. Someone, a couple of owners back, coverd the entire metal hull with fiberglass. It doesn't leak a drop, and while the glaas probabaly added some weight, it dindn't bog the motor down at all. It's usuall just me and my Lab in the boat. <br /><br />I let my son take the boat out the other day. I noticed that the stern sets much lower in the water than the bow. A lot of this is normal. Motor's on the stern and nothing but passengers are toward the bow. I noticed that the boat doesn't come completely up on plain at full throttle. My son put one of those fins on, and it helped the plain out a little. He says the motor needs to come up some. At full thottle, looking down at the motor in the back, you can't see the fin at all.<br /><br />Okay, so I get all that. The question I have is how much to raise it up. And does the fact that the stern sits much lower than any other boat I've ever had, need to be factored in to the equation? The motor seems to sit very low in the water by design. Could this be possible?<br /><br />Just for grins and giggles I pulled the rear of the floor up to see if the boat was maybe holding some water. But it's dry as a bone from bow to stern. <br /><br />Thanks for any help.

 

[rickdb1boat]

Re: How should boat sit in the water?

The cavitation plate should be even or within a inch or so from being even with the bottom of the hull. Sounds like it's too low right off hand if the plate is below the waterline when on plane. The reasons it sits so low in the rear could be the motor is too large and heavy for your type boat or the weight of accesories and such, in the boat is not evenly distributed. What type of Alumnimm boat is it and what size is it(Length and width)? How is it laid out?(Seating, helm, ect). Can you post a picture of it? Is there a motor capacity maximum rating sticker on it?

 

[Jack Shellac]

Re: How should boat sit in the water?

Those old motors were heavy, so it's probably normal for the boat to sit low in the stern. The aluminum boat is not heavy enough to offset the motor to any great degree. The anti-ventilation plate should be roughly at or just below the bottom of the boat. You can raise it up by degrees until you get the best performance. If the propeller starts ventilating [some call it cavitating] in turns, you've gone too far and need to go back down to the previous spot. You want it as high as you can get it without the ventilating. Also try shifting weight forward to see what works best. It could call for a prop change to get everything just right. Good luck. Enjoy your boat an welcome to the forum.

 

[HighPlainDrifter]

Re: How should boat sit in the water?

Oh, it's plenty big enough enough to hold that motor. I originally went and saw it while it was still in the water. The guy had a REALLY heavy 150 Johnson on it, and it sat in the water only two inches deeper than it is now sitting with my 55 on it. It's a fairly large hull. The fiberglass covering added strength to the transome. The rest of the hull is completely empty . . . well, except for the teleflex helm, steering wheel and cable; and the Motor controls and cables. Nothing else.<br /><br />As for my previously posted questions: Should you take how low a stern sits in the water into consideration when deciding on transome motor height? (Paraphrase)<br /><br />The boat doesn't handle badly, it's just losing about 900-1000 RPMs after my son put on the fin. I thought that you should at least be able to see the fin if you look down at the motor at full throttle. Even with the cav plate even with the boat, that damn fin is buried--can't see it at all.<br /><br />Any thoughts?

 

[ob]

Re: How should boat sit in the water?

What length and weight is this aluminum boat.If the previous owner had a 150 on it before you and you're running a 55 ,I'm thinking that your planing issue may be underpower related with respect to getting it on plane.That coupled with a possible stern heavy weight distribution problem that could be caused by the fiberglass overlay.<br /><br />With that said,lets just try with what you have now and see what can be done to improve the performance.What is the engines wide open throttle rpms now?What length and approx weight boat is the engine pushing?What diameter and pitch prop is doing the pushing?

 

[vaughneric2000]

Re: How should boat sit in the water?

OB:<br /><br />I don't really have the means to weigh the boat, and the weight is not on any of my registration paperwork. However, I think I found the problem. The boat has an area below the stern flooring that I didn't notice on my first inspection. I drained about 10-15 gallons of rainwater from it, and now the engine gave me back that lost speed. I moved the motor up on the transome until the cav plate was just slightly above the botton of the hull--one sixteenth of an inch. I moved the motor up as much as I could--only about 2.3 inches. The foil on the motor now rides just above the water at WOT, and the boat plains very quickly. <br /><br />However, when at WOT, I hear a subtle but distinct change in RPMs. Not much, really, but I don't know if the change in RPMs is normal, or if I am cavitating/ventilating (I'm not really sure of the difference). The motor is running so well, I'd like to leave it alone. But I'm worried amout damaging the motor. And even though I hear that change, the boat looses no speed at all in the turns.<br /><br />What do you think?<br /><br />Thanks, <br />Eric

 

[vaughneric2000]

Re: How should boat sit in the water?

OB:<br /><br />I forgot to say that I am not loosing rpms on the turns. And at 2/3 throttle, I do not hear that change in PRMs at all.<br /><br />Could the change be normal?

 

[ob]

Re: How should boat sit in the water?

Is your boat equipped with a tachometer so you know what your wot rpms are at?The engine should be propped to turn between 5500 to 6000.Nearer 6000 would be better.Hard for me to pinpoint the subtle rpm change you may be detecting at wot.The fact that the prop doesn't seem to ventilate in turns leads me to believe that the engine is not mounted too high causing the prop to lose bite at wot while cruising in a straight line.Could be carb related though.<br /><br />Try to post what your wot rpms are and the approx. length of boat and prop dia and pitch if you can so that we can get a better feel for how to advise.<br /><br />Glad to hear you found your stern heavy problem.