Need ideas to stop porpoising?

[Bushwacker]

I'd go 1" at a time and try it out. With engine trimmed as high as possible without porpoising, you should be able to make a moderate turn at cruise speed without ventilation/cavitation, although you may have to trim it down a little for hard turns. It will also help if your prop has a fair amount of cup in it as this seems to allow you to run a higher mounting position. Years ago I switched from an aluminum to an OMC SST stainless prop, and raised engine 1" as they recommended. (Engine was originally mounted same as yours with cav plate lined up with bottom of hull.) I picked up about 3 mph in top end, as was advertised. You may find that you have to run with engine trimmed a down a little more in rough seas to prevent ventilation or cavitation.

[gss036]

. Ok, looks like I did it!
[image][/image]

[McGillicuddy]

Just an unschooled observation but it looks like a lot of negative trim there. If you are running with the trim down all the way, I believe it may be contributing to the porpoising as it will cause the nose to dive, constantly fighting the bow's desire to rise and plane. I don't know the boat or motor but if there is an angle setting adjustment, hole you may want to set the shaft a little closer to vertical - I think the angle of the cavitation plate is the first thing to straighten out as it is the first "flap" that directs your glide in the water. All the wings, foils, and trim tabs are just inefficient counter measures if the motor trim is off. Experiment with the motor trim more.

[Fr. Frank]

The cavitation plate must be measured with it being exactly parallel to the keel. Your photos show negative trim angle. I suspect you will find the cav plate a bit below the keel and neutral trim angle.

The suggestion to raise it one mounting hole at a time is a good one, although I suspect you're going to end up raising it two holes. For an outboard, the ideal trim angle under way is with the cavitation plate perfectly parallel to the direction of travel with the hull at optimum angle of attack, and the rotating propeller blades just barely piercing the surface. Unfortunately, the only hull I've ever seen that ideal achieved with is an Allison XTB.

SO we compromise. Raise the motor, use a sternlifting prop, and play with trim angles.

BTW, a sternlifting prop is one which provides most of it's lift from the inner 1/2 of the diameter of the blade length. That's why progressive-rake 4 blades provide more sternlift than 3, and cleaver blades LOTS more sternlift than chopper or "big ear" props.

[gss036]

The negative trim angle look in the picture is probably because the tongue of the trailer is high to accommidate hitching up to the truck. I normally try to keep the top of the motor running flat with the water or a little higher.
Tom Clark in Seattle sent me the following e-mail(s)after looking at the pictures.
That is a Turbo prop made by Precision Propeller (now owned by Yamaha).
Those are great props on smaller boats, usually very fast,
but not a great prop for a high horsepower motor on a pretty big boat.
Looking at the photos, I would recommend raising the motor two holes (1-1/2").
From the Honda propeller catalog I would recommend the OFS 4 Blade
15-1/4" x 16", Honda part number: 58334-ZY3-B16CL
I had a an e-mail from the sales guy at Cap Sante and he wants me to bring him the boat this week and will raise the motor, now I will need to work on the prop issue. I have no idea what the price difference would be on the 2 props. Any ideas? He is off on Mondays, so will contact him on Tuesday after I go salmon fishing in the morning.

[Fr. Frank]

The OFS4 prop is actually made by PowerTech Props, and is, in my opinion, one of the 2 best props made for SeaCraft hulls. 16" pitch sounds perfect in a 4 blade. It is a heavy stern-lifting prop, very strong blades (not thin), with moderate progressive rake, allowing for a wide range of trim angles. It also happens to be the basis for the prop I ran on my 20' Seafari, although I ordered mine in a custom 3 blade 16.5" P and 15.25" diameter. I had a Merc V6 2.0L on my boat at the time, which was built to produce about 190/200 hp.

The other advice from Tom is about right. You need to get the cavitation plate above the hull bottom so that it doesn't pull the stern down while planing.

[eggsuckindog]

1 thing at a time, looks like 2 holes to me also, then get a WOT RPM reading so you can get the correct prop pitch the first time

[gss036]

They raised the motor 2 holes for me this morning and we ran it for about 1/2 hr. Did not get any porposing and and was able to get 5950 RPMS(range of WOT 5K-6K) out of it w/2 adults and 90 gal of fuel and the extra 60 lbs of lead in the bow, I just took the lead out as I don't think it will be needed now. I did not check the speed but plenty fast, had to dodge crab pots.
I asked about the 4 blade prop and they were not very eager to go there because of the $500+ cost. I even offered to split the difference on the price increase to swap the prop, but all I got got was no comment or we'll see.

[McGillicuddy]

Excellent, now get back to fishing!

[Briguy]

Gosh darn this is one of the best posts as of recent. All awesome, spot on information. What a great community. Kudos to all that posted. Sounds like the dealer is stepping up to the plate. Now, get the lead out!