Motor height, prop, and trim

[Mimic Shiner]

Just a couple questions, I have a 98 Ranger R61, 16.5 foot with a 115 horse mariner on it. When I got the boat the motor was located in the lowest setting and I used it last year like this but yesterday I moved it up to the highest setting to try and get a little more speed and to have the boat ride higher when I trim it out. I took it out this morning and it seems to have made no difference possibly even lost a little. It's got a good hole shot and when on plane at WOT I get about 38 mph and I when I trim it I can only get to around 40, which seems slow to me for this size boat and motor. I also notice water spray coming from pretty far forward so I doubt the bow is being lifted very much, if at all. I think last year on the lowest setting with the motor trimmed out I could only get about 42 mph out of it as well which too seems slow for the size of boat and motor. I should say, it has a 3 blade aluminum prop on it 12.5x23 and has a pretty good ding in one of the blades. I'm sure this affects it a little, but I would think when trimming it out I would pick up more than 2 mph. The rpms seem fine 5000 to 5250 which is where they should be. I'm wondering if a SS prop would make a huge difference or a jack plate as well, moving the motor back a little farther from the boat. I also put a keel guard on last year and that seemed to slow it down. I'm sure if trimmed out it properly it should be lifted out of the water but it doesn't seem like its lifting high enough. Any thoughts???

[mamohr686]

My brother (Prairiefire) used to have the same setup. He was running a 4 blade (renegade on a johnson) SS prop on his right at 50 mph. A 23 pitch seems too big for your set-up. Isn't the operating range of your outboard at 5500rmps? His boat was also trim sensitive, he had a midrange sweetspot and if it was overtrimmed he would lose a few mph. My advice would be to watch a gps and find where the speed is the highest, then check your rpms. Re-prop your boat based on that rpm reading. A smaller pitch prop is going to give you way better holeshot and midrange, based on your rpms I think top end should be better. A jack plate might give you some more bow lift and you could dial in a better holshot position, but on a smaller boat like yours a SS prop will make a bigger performance difference. On another note, weight was a huge factor in speed and lift in his ranger. Anchor placement in the bow, and a couple of 200 lb bubba passengers would drastically reduce speed. The keel guard shouldn't make a difference on top speed because it shouldn't have any contact with the water at W.O.T.

[hydro]

If the bow won't come up when you trim it out the hull may have developed a hook. Try putting a 6' straitedge on the bottom of the boat to be sure is is flat. If there is a gap forward of the transom that would be a problem.

While you are at it, push the straightedge back and measure from the hull surface down to the center of the propshaft. It should be about 4" or so down with that size prop. If it is, say 6", the motor needs to be raised up on the transom.

[Mimic Shiner]

I kind of thought the motor rpm range seemed a little slow too, but the book on the motor says 5000 to 5250. I'm also going off the tac in the dash so I don't know how exactly accurate that is. When I took it out yesterday it was only me with less than a half a tank of gas and no gear so I know it wasn't a weight issue. I put a strait edge on the hull, it doesn't seem to have any gap. Measured down too and it's right about 4 inces where I have the motor now. Where the motor was before it should have been around 7 inches so I imagine it was difinitely too low. It seems like now when I trim out though I can't go very far and the prop will start coming out of the water and I'm geussing I'm getting no downward push to lift the bow. I am still wondering about a SS prop, but would it make that big of a difference on the trim? Or am I better off lowering the motor back down a few holes to try and get more prop in the water? Mamohr do you remember on your brothers where on the transom the motor was connected, or did he have a jack plate, and what the pitch was on his prop? It just seems to me when I hit the trim the effect is very minimal and I would assume on this setup I should get a much bigger difference than 2 mph.

[mamohr686]

My brothers boat ran very flat also, it's the hull design on your model. He didn't have a jackplate, and his Johnson was mounted in one of the middle holes from the factory. I think the prop was either a 19 or 21 pitch and it ran right at 5500rpms at W.O.T. It threw a small roosertail about 6ft tall when it was running at the higher speeds. Find a dealer that would switch out a prop if it doesn't run right on your rig. If you can't find one locally, do a websearch on kenyon powerboats and give them a call. They'll ship any merc SS prop (besides a fury) for $400 and if it doesn't run right you can swap it out (but have to pay for return shipping). Try a 4-blade Trophy plus in a 21 pitch, you will not be dissapointed. What does the book say the max rpm's are? You might be looking at the operating range where the boat makes 115hp.

[marine_man]

I'd lower your motor down one hole and see how it runs...

marine_man

[boatfixer]

I'd forget about any particular measurement and place the motor so that the anti-ventilation plate is even with the keel of the boat. Many times we get focused on what hole it needs to be mounted in. Whos to say the holes were drilled into the transom in the right place to begin with?

Lets say right now it is even and its ventilating when you trim out. Look to a ss prop with a higher rake and more than likely going to a 21p.

What if its too high? Drop the engine down and look at a 4 blade 21p ss prop.

Unfortunatly it's not a 1 size fits all and some trial and error is most often what it takes.

[hydro]

If your propshaft measurement was at 4" and you get blow out when you trim up, then the setting is too high for the prop that you have. Drop it one hole and see if things improve much. That's the inexpensive route.

There are wo things will help you here. Since your RPM's are just over 5000 at WOT, you have too much pitch for the combination and your 115 HP motor just doesn't have the ponies to spin that prop. As mentioned above, you need to be at 19" to 21" max pitch. The second factor is your aluminum prop. Switch to a stainless 3 blade prop designed to give you "bow lift". Stainless props have thinner, stronger, cupped blades and are more efficient at higher speeds. A cupped 19" SS prop and an extra 400 RPM will do wonders for your performance. Do your research with a propellor dealer here to make the proper selection, and avoid too much HSOforum advice.

You should see a combination of things happen with the right stainless prop. First a prop designed for bow lift will be better able to hold a grip on the water at higher trim angles, lifting the nose of the boat. Second, with a lower pitch you will see the RPM at WOT pick up, ideally to about 5400-5600 RPM when trimmed just enough to keep the bow up. When trimmed too high it will start to slip, throwing a nice big rooster tail, but you will lose speed as your RPM's creep up past that 5500 RPM sweet spot. Third, you will notice that you can run the motor higher on the transom without the blowout you presently see as you trim up. All things together you will probably pick up maybe 5 MPH as well. Now balance that against the $350 - $500 that a good SS prop will cost and make your decision. Happy experimenting.

[marine_man]

I'm pretty certain his 115 is only rated for 5250 at WOT... be sure to use that as your top end.

marine_man

[hydro]

If the motor is rated at 5250 RPM WOT then use that as the centerpoint in my post above. The idea is the same. If you spend a lot of time using the boat at the lower speeds, buy a second, lower pitch prop but watch the RPM as you will easily over rev the motor if you open it up.

Ideally, to operate at 10-15 MPH, a pontoon boat is your vehicle of choice.