How do I get more power from 90hp Johnson?

[underwater]

I just bought a 17' king fisher with a 82' 90 hp Johnson outboard on it. The thing runs great and my top speed is about 40 mph. Is there any thing that I can do to get a little more out of it? Maybe a new prop? or something of that nature?

 

[clanton]

Re: How do I get more power from 90hp Johnson?

How many rpms at wide open throttle?

 

[JB]

Re: How do I get more power from 90hp Johnson?

Hi, Underwater. How many thousands do you want to spend? Your best bets are probably with hydrofoils, hull maintenance and, as Clanton suggests, prop selection. Trim tabs also help. Engine modifications almost always cost much more than they are worth. If the rule of thumb for autos is that added horses cost $100 each, for outboards it should be $1000 each.<br /><br />Good luck with hull and prop fine-tuning.<br /><br />Red sky at night. . . <br />JB

 

[Bob_VT]

Re: How do I get more power from 90hp Johnson?

The basic result for getting more power is caused by the weight loss in your wallet.<br /><br />I have a (heavy 1000 lb hull weight) 15' Steury with an 1979 85hp Johnson. My top speed with me driving is 40.5 mph (by GPS).<br /><br />My previous top speed was 38.7 mph until I spend HOURS lightly sanding (600 grit) my hull. So what my quest for speed resulted in was just about a 2 mph gain. I really don't think it was worth it considering I don't race the boat.<br /><br />I did discover a slight downside to my venture too. My boat now slips more in a very hard WOT turn. It seemed to have lost it's bite on a turn.<br /><br />Bob

 

[underwater]

Re: How do I get more power from 90hp Johnson?

At top speed, my motor is turning at 6000 rpm. That is if my tach is right. You see when I bought the boat nun of the guages worked and as I fixed them one by one I noticed that the tach was set on 6 cylinder I put it on four. I did not know if it reads diffent for the two stroke motor.

 

[clanton]

Re: How do I get more power from 90hp Johnson?

The numbers on the back of the tach are not for number of cylinders, they are for pulses from the stator. I would move it back to 6.

 

[clanton]

Re: How do I get more power from 90hp Johnson?

The numbers on the back of the tach are not for number of cylinders, they are for pulses from the stator. I would move it back to 6.

 

[kev_79]

Re: How do I get more power from 90hp Johnson?

The tach does read different for a two stroke and a 4 stroke, but if it has always ran a 2 stoke, you probably won't have a problem. At 6000, you are running, I think, on the upper end of the limit, maybe over. If you prop it up about an inch or two on pitch, you can probably run about 5+ mph faster, but you will lose a little on the bottom end. Try to keep it at about 5500 or so, and if you are willing to give up the take off, you can go faster. JB is right, if you want more hp, it'll cost you. If you wanted to make it a 105hp, you could probably go buy a 115 for about the same price.

 

[Dhadley]

Re: How do I get more power from 90hp Johnson?

First get the tach set correctly (on 6) and see what kind of top RPM's you get and what water pressure. Once you get this info tell us the make, style, diameter and pitch of the prop. Also the exact speed and X dimension and we can calculate percentage of slip. <br /><br />Good luck!

 

[evin300]

Re: How do I get more power from 90hp Johnson?

Right on Dhadly! I dont think the "slip" word is used nearly enough, I mean, Its the only way to gauge the efficiency of your Hull/Prop/Engine set-up. Also I think that we need the gear ratio of the Lower Unit, correct? <br /><br />Bob_VT That is an inredible gain of MPH! Thats the cheapest 2 MPH you will ever gain in a boat. If these are good consistent numbers, I am quite impressed. I have always wondered about how much could be gained from sanding. I would guess that faster boats may gain more, and slower boats would gain less.<br />What kind of prop are you running? Your hard turn thing sounds like a prop issue to me.

 

[evin300]

Re: How do I get more power from 90hp Johnson?

Bob, also were not just talking 2mph of speed, but greater overall effiency, which = gas savings=

 

[Bob_VT]

Re: How do I get more power from 90hp Johnson?

13 X 17 Cupped alum prop. I also have a 4 blade composite 13 x 17 which is a bit slower but the MPH increase was almost the same. No the boat used to turn and bite at WOT with both props, now it just skips and slides a bit.<br /><br />The overall sanding was combined with filling chips from the previous owner with fiberglass resin. It was ALLOT of work and I did it on my back. Overall I was very impressed with the results and considering I trailer my boat.... the bottom stays very clean.<br /><br />

Bob

 

[FredWinNH]

Re: How do I get more power from 90hp Johnson?

OK,<br /><br />here's an off the wall idea...<br /><br />Any golfers out there would know that the reason a golf ball is not completely smooth has to do with aerodynamics. A smooth surface causes too much surface friction. Early ball designers found that by making the surface irregular with a pattern of dimples, a laminar layer of air would be slowed at the surface of the ball which has the effect of reducing the friction of the ball traveling through the air. Since air is a very thin fluid, why wouldn't this same concept work on boat hulls traveling through a more viscous fluid, water? Perhaps the dimple size and/or pattern would be different, but why wouldn't that give even less friction than a completely smoothly sanded hull? Anyone?

 

[kev_79]

Re: How do I get more power from 90hp Johnson?

That is a very interesting idea Fred, but I am fairly certain, it won't increase your speed. The problem is that the reason that a golf ball travels farther, is as you stated. But those are laws of "aerodynamics", and even though air can be seen as a fluid, it doesn't apply to a boat on top of the water. The two ways air flows around an object are laminar and turbulent. I won't go into detail, because you will probably get tired of reading. If you do really wanna know, write me back, and I will explain them. But anyway, the key term is "flow around an object". I do think ,though, it is a great idea, and has very high potential on your submarine. Get back with me if you have any discrepencies with what I have stated. You might be right, and then we can make some MONEY!

 

[evin300]

Re: How do I get more power from 90hp Johnson?

So is it safe to say that the #600 sanding scratches are de-laminating the water?

 

[Bob_VT]

Re: How do I get more power from 90hp Johnson?

I was told that sanding the hull creates tiny scratches that disrupt the water flow that carries air from the bow to the stern. My first attempt was initially to wax the hull however this caused a loss (small but measuable) in speed. The waxed hull slid through the water and the sanded hull has enough disruption to make the hull ride over the water.<br /><br />Some racing hulls (high performance) have a slight curve at the end of the hull to create a disruption and make the boat ride higher at speed.<br /><br />The golf ball idea is good however I know that water acts and reacts much differant than air. I agree with Kev and gee I would love part of that money venture.<br /><br />Bob

 

[evin300]

Re: How do I get more power from 90hp Johnson?

Bob, Have you tried Stainless? Drop 2" pitch to match existing RPM when going from aluminum. You can also raise the motor with a SS prop. Propeller and engine hieght are next (and final) on your list for efficiency gain and overall performance per dollar spent. Expect another 2-4 mph depending on your particular set-up. Also consult a with a pro. What slip% are you running now?

 

[evin300]

Re: How do I get more power from 90hp Johnson?

Bob, Thanks for further info. on Sanding vs Waxing, as this sounds familiar to a article I read a long time ago. Were sanding strokes uniform? Parallel with length of boat? Could you give a guesstimate as to the combined area of the gelcoat chips repaired, as to the area of V-shaped high speed surface of the hull in percentage terms? Thanks, Mike

 

[oldevinrude]

Re: How do I get more power from 90hp Johnson?

In regards to water vs. air drag, think of a propellor driven airplane of 100 HP. It will easily travel 115 MPH. Now think of a boat with 100 HP. The key is to get the hull out of the water and into the air as much as possible. Hull design, trim, and raw horsepower all get the hull into the air and out of the water. The dimpling and "laminar flow" ideas have been tried with limited success. Appearance and maintainability always win out unless racing. A sharks skin is roughened for the very purpose of water drag reduction. I am obviously no expert on hydrodynamics but have done some experiments over the years with this topic. Keep the ideas coming, but remember, water and air have not changed in the history of intelligent thought. They are well marked trails that should be researched prior to investing time and money into an idea that has been tested and retested for years.

 

[Dhadley]

Re: How do I get more power from 90hp Johnson?

I know we've kinda gotten away from the original question about the motor and have been looking at hull configuration. Here's what we do on V bottom boats -- make sure everything is straight and sharp. We go forward about 2/3 of the length on the running surface and make sure it's straight. How far on each side of the running surface depends on what you want to accomplish. As you take hook out of the bottom you will take longer to get on plane (increased bow rise is common) but have more top end. Unless it's a race boat we straighten the V or pad and the bottom out to the first lifting strake. We take 1/2 the hook out of that strake and leave the rest. <br /><br />Then we go to the edges and make them as sharp as possible. The edge of the pad, the step and transom edge all get sharpened. Sharp edges release water quicker than "rounded" edges. <br /><br />How well does it work? Let's say it's 2 mph. On a flats or bass boat some would say it's worth it, some would say it's way too much work. On a race boat 2 mph may be a gift from heaven. <br /><br />Then we start shaping the gearcase. But that's a whole other story. Just like props. Hope this helps.<br /><br />Good luck!