TOPIC: Stability on plane

I'm a newbie to running powerboats, so I'm hoping you guys can help me with this (or just tell me to suck it up, Buttercup.)

The Chaparral 15 tri hull I have seems pretty unstable on plane. Max speed is about 30mph with two of us aboard (we're not light) and it will generally stay on plane down t about 20mph. When it's up on plane it seems to move around a lot, kind of side to side but not really chine walking. It doesn't porpoise either. It kind of feels like it could snap steer left or right, almost like it wants to fishtail like a rear wheel drive car.

I think there are several factors at work, but I may just not be familiar with how small old boats normally feel.

The boat has an 85HP Rude on the back, 15hp over max rating. The motor weights about 250lbs. I have a massive battery and (2) 6-gallon fuel tanks in the cubby at the transom. We normally run with just the two of us and no one in the tiny bow seats. The boat sits lower at the stern than I think it should on the water, but not dangerously so. Basically I think it's loaded too heavy at the rear, but there's not really a good way to get more weight forward due to lack of storage space - I can't really move the tanks or the battery up there. It takes a minute for the bow to come down when getting on plane but it doesn't seem overly slow.

The boat also has a tendency to list to port while on plane but not at rest, even though I'm seated on starboard and heavier than my navigator. Coincidentally, part of the anti-cavitation fin on the motor's LU is broken off on the port side as well. Could this be causing the list or adding to instability?

Anyway, I'm wondering if an add-on hydrofoil for the outboard would stabilize the ride and perhaps correct the list at speed? Or if maybe a set of trim tabs on the hull would be a better option? Just for reference, there's no power tilt/trim on the motor so I always run the motor full-down.

You have too much hull in the water if you are trimmed down all the way. Get the bow up.

really? How does too much boat in the water make it less stable?

(Not disagreeing, trying to understand...)

EDIT: I've been doing some internet mining based on what you said, and I think you're talking about bow steering? Having too much bow down in the water at speed caused the bow keel to catch wakes and pull the boat left/right, and can even cause a spin/flip. This does sound like what I'm experiencing. I'll try next trim hole out on the outboard and see if it helps/ hurts.

Any other thoughts from the rest of the assembled?

Yep, trim it up and get the bow out the water...

ultraclyde wrote:

The boat also has a tendency to list to port while on plane but not at rest, even though I'm seated on starboard and heavier than my navigator. Coincidentally, part of the anti-cavitation fin on the motor's LU is broken off on the port side as well. Could this be causing the list or adding to instability?

Anyway, I'm wondering if an add-on hydrofoil for the outboard would stabilize the ride and perhaps correct the list at speed? Or if maybe a set of trim tabs on the hull would be a better option? Just for reference, there's no power tilt/trim on the motor so I always run the motor full-down.

The list to port under power is due to the torque of the motor. A right-hand rotation prop (clockwise from the stern) will be trying to spin the boat in the opposite direction (anti-clockwise from the stern). Adding a hydrofoil will help some but probably not enough. But adding trim tabs will fix it. Note: powered tabs are adjustable under way and work really well but are expensive. A single fixed tab well out on the port side transom would work too and is much cheaper. Downside is that it could take several tries to get the right angle (for cruise) and it would be doing it's thing at all speeds.

-Jerry

ultraclyde wrote:

really? How does too much boat in the water make it less stable?

It's a phenomenon known as "bow steering". I found an excellent description at another site:

"Bow steering occurs when the bow is too flat in the water at high (planing) speeds. The chines will "grab" and pull the boat to one side, often when encountering a wake from the side."

So, to cure that you need to do as suggested, move the tilt pin "out" so the motor will tend to push the bow higher. This gets the front end out of the water, reduces drag, increases speed.

Typically you'll want the water spray to break at around the driver's seat, but every hull is gonna perform differently so you'll have to experiment for best ride/performance.

Just move the tilt pin "out" one hole and see how it rides. Move it out one more then see what happens, you'll know soon enough if the motor is tilted-out too far, as you'll get the bow too high on takeoff and/or on plane.

HTH.......ed

...other issues maybe a soft transom where motor would tiltbin under load and push bow down or your steering cable may have lots of play causing the motor to oscillate left and right.

Frankly you could probably solve your problem simply by replacing the motor with a motor matching the rating. An 85 HP weighs far more than the 70 hp motor the manufacturer rated it for (not to mention that most states have laws making it illegal to exceed the rated HP). It has too much torque and is trying to make the hull go faster than it was designed to go. All of this is contributing to the problems. Tilting the motor out will bring the bow up some but it won't fix the problem of too much weight in the stern. There are reasons why boats are rated for certain size engines and weight on the stern is one of the factors.

These boats normally run relatively flat and don't chine walk or bow steer. But when overpowered they can do some very strange things that can be dangerous. If they bow steer and hit a slightly larger wave it can cause the boat to spin out and rotate on the bow tossing everyone in the boat, into the drink.

PS: I looked it up and an 85 weighs anywhere from 100-125 pounds more than a 70 HP O/B.

An 850-4 Merc longshaft weighs approx 252 lbs.

Please post a few pictures showing the height of the cavitation plate in relationship to the bottom of the hull.
Hold a straight edge (or long level) held firming against the bottom of the center of the hull with the aft part of the level extending aft enough to show its relationship to the motor's cavitation plate, and have a pal snap pix.
Also,...Since you are at the helm on the starboard side, but the hull leans to port underway,...Perhaps all you need to do is reset the aft edge of the anode (trim tab) more to port???
doc

Sorry guys, I was out of town for a week.

Ike - I'm sure using a lower horse motor would be fine, and you are correct about the ratings being there for a reason. However, this is $1500 boat. I I'm going to the effort and cost of motor replacement, it's not going to be on this hull. This boat was bought new with this 85 and has survived family life for 40+ years that way. I suspect it will be okay. To quote your signature "Don't tell me that I can't, tell me how I can." OTOH, if some has a good running 70 JEGO with controls they want to swap for an 85 in Georgia, I'm all ears.

I'll have to take some pic this week. I checked it a while back and I think the cav plate was about even with the keel, but a photo will tell for sure. The trim tab is just barely turned to port - almost straight.

My plan is to take it out to the lake this Sunday and try a couple different tilt settings. I suspect that will solve the problem of the bow steer, but we'll see. I'm a scientist by trade, so I prefer to change one thing and test, then repeat until you find the thing that solves the issue.

I am interested in the answer here because I have a similar issue with a boat that has a motor within the rated HP.

Actually, a quick check of NADA Specs shows the 85 hp motor at 300 pounds and the 70 at 230 pounds. With this in mind the weight / horsepower ratio would seem to indicate the 70 would give you better speed and performance than the 85. Lets face it adding 15 hp to push 70 more pounds seems a bit off when referencing performance.
I would agree with the others that the trim needs to be dialed in to get the hull out of the water for less drag and better handling. Good Luck.

I have to wonder which 85 HP Merc and which 70 HP Merc are relative to your weight listings.
I can see where the 85 HP '63-'64 850-6 cyl, and the 70 HP ''60-'61 700-6 cyl, "might" fit your criteria, but the '70s 85HP 850-4 cyl and the little '70s 70-3 cyl would only weigh that much if they were dragging a v4 OMC as an anchor!!!

doc F

just for reference, the '73 85HP Evinrude that's on the boat is listed in the advertising I found on this site at 250ish lbs, 252 I think.

"just for reference, the '73 85HP Evinrude that's on the boat is listed in the advertising I found on this site at 250ish lbs, 252 I think."

The '73 85 HP Evilrude is a very heavy v4.
A closer weight comparison would be to compare the '83 70 3 cyl Evilrude to an '83 85 HP Merc, or better yet, compared to a '79 70 HP 3 cyl Merc.

You should understand that I am Phantom Black to the core, for a lot more reasons that weigh advantage.
Doc Frankenmerc

From what I see in the picture of you with the level, that's a Vee Bottom without a pad. So on a plane it will want to fall over one way or another. Trying to get pure V bottoms to stabilize is a real challenge. Not saying it can't be done, it's just not easy.

So I'd prolly lower the motor a bit because it seems you are getting air between the lower V and the leading edge of the LU and the cav plate. This can exasperate the torque effects.

I'd also try a good cupped 4-blade to even out the blade "bites" in the water. The 4-blade should add some stern lift so it will allow the boat to run flatter even with the motor trimmed back.

Is there no way to add a battery "hold" in the deck along the center-line? Would be really nice to get that weight low and directly on the center-line and forward of the transom ...

I also agree that hydraulic trim tabs will be your best bet to trim on the fly

Don't pay too much attention to the Murk Heads. They think black is good. But OMC thought the shorter cranks and lower power-heads were more desirable and I think in your situation that's good planning. You're already trying to balance a boat and motor on an edge ( V ). No need to put weight higher ...

I have no problem with the 1969 Merc 1000 being too heavy on my G3!! Oh by the way, the pic here is at 55.2 mph and had more, but gets a bit squirrely at higher speed!! No pad on my G-3 either. This motor weighs 228 lbs I'm told.

Oh yea, I am a merc head, but I owned the other G-3 shown here from 1960 to 1975 with that gosh-ugly v-4 75 hp 1961 Johnson and this ole Johnson at 75 hp WEIGHED 238 lbs. plus!!! it ran 48 mph with my set-up.

And I agree with Dr. Frankenmerc, the white engines do make great anchors!!!!

Can't help it that my wife has a 1960 Glasspar Tacoma with a 1960 40 hp White thing on it. (She likes to go slow!!!)

Back to the topic---Trim that motor up more, get the bow up and enjoy!!!!


Jim Savage
Crawfordville, Florida