89 cobra

[jimmyboy]

I have a 89 bayliner cobra with a force 150hp how fast should it be going on the lake? It's going about 35 mph is that normal or should it be going faster.

 

[Frank Acampora]

Re: 89 cobra

Speed is always a subjective thing. It seems that every one else is always going faster than you. And funny thing--every time I ask someone with a Chrysler 75 how fast their boat goes I always get the same answer "OH, about 40."

Boat speed varies with hull design, engine, prop, boat load, and trim. As an example, I have a 21 foot cuddy that gets 38 MPH top speed with a Chrysler 140---but I have played around with engine height, prop pitch, and trim. A little more experimenting this year, and I should get it to 40.

If your boat is less than 20 feet long and lightly loaded, I would expect a little more than 35---But you need to optimize props. The stock black aluminum prop is a piece of CRAP. You will gain at least 2 MPH by doing nothing but replacing it with the same pitch in a nice Michigan stainless. I gained one fellow 7 MPH with just a prop change--but he did have incorrect pitch on his prop.

However, You need to know not only the top speed but what RPM the engine is turning at that speed. Wide open throttle should be somewhere around 5000 or better. I am not sure about the 150, but most other Force engines are rated to run 4500-5500 so the 5 cylinder should not be different. Only by knowing the WOT RPM can you make an intellegent guess as to prop pitch.

YOu need to also experiment with the power trim to find the sweet spot where the hull it lifted highest out of the water without the prop ventilating and cavitating. And you need to experiment with internal load--anchor, porta potty. life vests, batteries, fuel tanks if portable, line (rope) etc. all that weight adds up and you would be surprised at how much it can be.

So, you see: while your question is simple, the answer can be quite complex.

 

[jimmyboy]

Re: 89 cobra

at w.o.t. i was going 35 mph with rpms at 3500 max. I did have 3 other people with me we probably wieghed 500+ pounds total. but my rpms still should of reach 4000+ ?

 

[pnwboat]

Re: 89 cobra

I have a 19ft 88 Cobra bowrider with a 125 Force that I picked up last year. It came with a 13 X 17 prop. I was turning 5600 RPM and going 40 MPH. Sounds like you're not turning enough RPM's. Maybe too much prop or something amiss with your engine. I had some issues with my engine when I first got it, but due o the wealth of information and helpful individuals on this site, I was able to correct and am very pleased with the performance.

 

[Frank Acampora]

Re: 89 cobra

Yeah! RPM should be in the 4500-5500 range. Running with RPM too slow in WOT is lugging the engine. This causes carbon deposits to form. Outboards are designed to run at higher RPM.

Figuring a 21 pitch prop and a 1.78 gearcase ratio, I get a theoretical top speed at 3500 rpm of 35 MPH. Given the inaccuracy of speedometers, this is exactly what you are doing.

So, if your prop is indeed 21 pitch, then you have too much prop for the boat. Dropping down to a 19 pitch should increase RPM to 4000 and a 17 pitch should increase it to 4500. Stainless will run 2-400 faster than aluminum.

To find the pitch of the prop look on the hub between two of the blades. It may have a number such as 13X21 stamped there. Also look inside under the prop nut and washer the prop number will be cast or stamped there and the pitch may also be.If you can only find the prop number, ask over on the prop forum. They'll know what it is.

By dropping down in pitch, the engine should rev up more and there might also be an increase in speed over what you are doing now. However, given the price of props, It would be best If you could find someone to loan one to you for test purposes prior to buying.

The question still remains though: Are you trimming up the engine for maximum speed? If the lower unit is tucked too far in towards the transom, the bow of the boat will ride low, more hull will be in the water and drag will increase. Speed will be low. Trim the engine up or out until it starts to gurgle (ventilate or cavitate) and the engine revs up. then trim down until it grabs water solidly again. This is the best trim position.

 

[jimmyboy]

Re: 89 cobra

what do you mean when you say to much prop? I've only had my boat for less than a year and have had it in the water 3 times. I really don't know how to adjust the trim, i adjust it until the boat is completly level if it's not adjusted properly will that cause it from getting higher rpms?

 

[Frank Acampora]

Re: 89 cobra

OK-fair enough. The diameter of the propeller for that engine is fixed by design. Any propeller you buy will have approximately the same diameter to within say, 1/2 inch. Pitch of a propeller is the theoretical distance it would travel forward through the water with no resistance or losses. So, a 15 pitch propeller would travel forward 15 inches with 1 revolution while a 21 pitch would travel forward 21 inches with 1 revolution. This is kind-of like the gears in a car. 15 (low pitch) would be sort of like low gear--real good acceleration but not so fast. 21 (high pitched) would be like overdrive--poor acceleration but top speed is faster. PROVIDED the engine has the power to turn it. Now, this is a very simple explanation and does not cover anything but the basics. There are other factors to consider but that is too much information for now.

So, when I say you have too much prop, it means you are using a prop that has too much pitch for the engine to turn properly. The engine can not turn fast enough to develop its maximum horsepower. This is, of course, assuming there is nothing wrong with the engine. That is why I figured out a theoretical top speed at 3500 RPM. The theoretical top speed told me that since you are close to it, there is nothing major wrong with the engine.

Now, as I explained before, if the trim is improperly adjusted, this would be part of the reason for low RPM. The more of the boat that is in contact with the water while it is "On Plane" that is, riding on the surface, the more resistance it will have to forward motion. Thus it will take more horsepower to go the same speed and of course, top speed will be lower than normal.

Look at pictures of bass boats at speed. They are very fast (by design) and run with the nose (bow) pointed up a little. They also ride on only about the last two feet of the hull. Now this is not possible to the same degree with a multi use boat like a family boat. However, You should still trim the engine to the point where the bow is riding slightly high--not level. and if it is a bow rider, move several people from the front to behind the windshield--you will notice a difference in speed.

As I also said before, the easiest way to determine best trim position is to trim the engine out (or up) away from the transom until it starts to cavitate. Do this while the boat is traveling straight ahead. Now, trim back in (or down)just to the point where it stops cavitating. This is usually the best and fastest running position. It will, however cavitate in turns so when you turn the boat, you must trim (in) a little more, then trim (out) as you come out of the turn.

If with proper trim you still do not get to proper RPM (which I suspect you won't) then you need to investigate using a lower pitched prop.

 

[pnwboat]

Re: 89 cobra

Something else to be aware of. I had one of those horizontal "whale tail" fins mounted on my motor by the previous owner when I first got it. Took it off and gained 8 MPH. It kept the bow down so that I was more or less "plowing' through the waves. Once I took it off, the bow would come up higher and boat would skip across the waves. At the time the fin was mounted on the motor, my trim was not functioning. I've since fixed the trim, but I haven't put tried the fin with trim.

 

[jimmyboy]

Re: 89 cobra

I removed the prop nut and washer and there were no #'s there was something that looks like a big nut but looks like it requires a special tool.