Hi everyone. I have a late 80s 88hp Evinrude SPL mounted on an OLD 15' Arrow Glass tri-hull fishing boat. I installed a tach and speedo and am a little disapointed with the readings. I am just barely getting 5000 rpms, if that, at WOT. And aren't even hitting 30mph (Its more like 25-28). I don't know anything about props, and don't even know what type or size, etc. that this motor needs. Also the one thats on it has a few nicks and scrapes and could be replaced.**Also, the motor has power tilt/trim, and these readings were taken when the motor was trimmed to optimal operating setting.Can anyone tell me what size and pitch of a prop this motor takes? Also what kind of prop would you all recommend? Am I correct in thinking that the nicks and scrapes on the existing prop could be the culpret?Thanks everyone, any info will be greatly appreciated because I do not know much and am new to the boating world.
5800-6000 wide open throttle, and the prop is a long-shot, but a 13-1/4X17" should be close.Is the engine mounted directly on top of the transom????, and where is the cavitation plate in relation to the bottom of the boat??
There are 4 holes on the transome bracket. It is mounted on the 2nd hole from the top, so it is one hole from sitting on the transome. The cavitation plate appears to be even with the flat part of the center V.
OK, that being said, you can go up one hole at a time-relatively easy to do-and test run.With the Cav. plate being flush with boat bottom in the second hole position, you may verywell be able to go to 4th hole, but one at a time, first.You should see about 125RPM per hole, some see more. You will find also, that less trim is required, and if you get to the point of ventilating, which it may in the 4th hole, you might have to stay in 3rd hole.Edit: According to the figures you give, a 15" pitch would fit your numbers given....problem is, I think you should be able to spin more pitch than that unless your boat is extremely heavy, or loaded with parasitic drag (un-clean bottom), or, possible engine problems. Anyway you can do a compression check????
I checked the compression when I bought the motor a couple months ago. The motor is still running just as good as it was when I bought it, its just that I didnt have gauges so I didn't know the actual numbers.The boat is a small, light 15' boat. So I was kind of expecting to hit maybe 35mph or so. Is this unreasonable?
Not unreasonable at all....that's why I'm concerned as to the numbers you gave, as they indicate a 15" pitch prop being used. On that boat, being light as you say, you should get an easy 6000RPM with that set-up.What type of speedo do you have??Does it "seem" like your speed is low??
With this being my first boat, I really don't have much to compare it to. But it does seem to be moving a little slower than I'm guessing that it should. I am not sure as to the manufacturer of the guages. It doesn't say on the face (I bought them at a wholesale shop, open box).I have a funny feeling that the prop that the guy I bought it from put on there might not be the correct one, and also that it has a few nicks and stuff as if it was scraped on a boat ramp while spinning. That is why I was wondering what prop specs to put on it so I can try a new prop. Theres just so many props avail, that I get confused on which one will work best.The boat accelerates and planes great, its just that top end I am concerned with.Thanks for your help.
Let's not get too excited yet. I'm not suprised as to the excelleration you're getting, I am, however suprised to see the speed as low as you are indicating.It could be possible that your speedo is off some, but figuring 5000RPM, with 2:1 gear ratio, figuring 14% slip, and using the 28mph figure, that comes out to 15" pitch prop....yes you will loose some effeciency due to minor nicks, dings, and what-not....Hummmmmm.....I'm still thinking something isn't adding up, here.....If the tach was wrong the actual could only be lower, which would indicate to me, engine lack of power.My neighbor runs an 85HP on his 17ft starcraft V-hull, and gets 6100rpm with a cupped 15" stainless prop...He does not have a speedo, but that's not an issue here.maybe someone will step in with some other ideas here, but I think you have an engine issue...Edit; The thing is, if it was as simple as recommending a prop...I would. Problem #1 is if what you have IS a 15" pitch, and IF I figured right, I can't say you need less pitch to get your R's up because that 15" is kinda "known" as a load pushing, high rpm prop, on the crossflows of the era, and #2, If your 88 won't spin that prop but 5000rpm, on a light boat...there is another problem somewhere. There is either something creating drag on your boat, or the engine is not performing to specs......
OK, I just pulled the plugs. They are pretty black. The contacts themselves didn't seem to be extremely dirty, although they definately need to be changed. There was however a lot of buildup around the base of the Threaded part of the plug. Not all around the threads, just at the very, very bottom. Does this sound like normal spark plug wear? Obviously they need to be changed, but does this sound like common characteristics?
No reason to be afraid.....the color of your plugs does not indicate a problem....The black is typical, the slight build-up is normal as well. I wanted to make sure you didn't have one caked full of carbon, or lead, or one white in color. We can set that aside for now.Next idea...this one learned from the "great one" we call him Lay a ruler along the hub of your prop (Fore-n-aft) and get the measurement from where the leading edge of the prop blade is, to the trailing edge (distance back).......
The prop situation is what we will determine with the measurement, but I think you have an older 17" uncupped prop that has a bit more slippage than a cupped prop and will result in lower speed, but "usually" higher RPM....Let's figure out what prop ya got afor we decide on the next prop.....
OK, I measured. I wasn't sure if you wanted me to measure from where the leading edge connects to the hub, to where the trailing-edged connects to the hub; OR if you wanted leading-edge, to where the trailing-edge hangs towards the back the farthest. From base of the leading edge to BASE of the trailing edge it is ABOUT 4 inches.The fin of the trailing edge swoops out a little and then back in to where it connects to the hub, so that measurment is about 4 1/2 inchesPlease forgive me if I confused you, i kind of counfused myself.
James35,I would not get yourself scared yet, the black you are seeing on the plugs and cylinder heads could very well be fixed by decarbing the motor and changing to new plugs. This costing you a grand total of about $14 dollars. I would start here, and get some solid numbers on your compression before you start getting worried. Make sure you are using a quality oil and a 50:1 mix with good gas.
James, I'm thinking you've got a 15" prop there, based on your measurement.I'd like to call on Dhadley for this, but I still feel there is an issue here we are missing.Is the bottom of your boat nice and clean-no build-up or anything such as that???Yes, always use fresh gas and good oil as stated above, but you said the engine runs the same now, as when you bought it....do you remember any numbers on compression????
I do not remember any numbers on compression. The bottom of the boat however is clean, its used in freshwater and is trailered, not docked.What would the effect be on speed/rpms if I move to a 17" pitch prop?Also, I always use 50:1 oil and 89 or 91 octane fuel.
James35,Before you start with props, consider getting a service manual and complete a Link and Sync. This way you will at least know everything is opening and closing when it should be. this is not a hard task to accomplish if you follow the steps in the book.
Well, in guessing you have a 15" pitch, if you went to a 17" pitch, that would drop your RPM about 400, and we need to get it up about 600-800, where it should be to run as intended.In this case, I don't think you would see any speed gain with a 17".I don't think this would be an issue here, but use 87-89 octane as anything higher only causes problems heat/power-wise.Edit: Do you have any problem with idle speeds, in gear or other-wise??
walleyehed + harpjames35 If I might jump in here for a sec. You should be able to turn a cupped 17" prop on a 15' runabout with no problem and at least hit 5500 RPM's and 40 MPH with that motor. Heck, my sis-in-law has a 24' toon with the 88 on it, running a 14" X 13" prop and it hits 5600 RPM's.Something is amiss here. I would try another prop and go with at least a 17" pitch. It Can always be re-pitched to a 15" or 19" if nessesary. Also, for a comparison, I had a 1967 Merc 65 HP turning a 17" pitch prop at 5500 RPM's on a 15" Wellcraft runabout and it hit 37 MPH. So, I would have to assume you would do better with the 88HP on yours. According to the prop selector, you should be running a 14" X 21" pitch for your size boat with a light load. My Wellcraft is a pretty heavy 15'er, hence the 17" pitch on the 65 HP motor. Either your Tach is wrong, the speedo is wrong, the prop pitch is wrong or the motor is not up to snuff. Try to borrow another tach and see what it reads. That's what's important right now. Then you can determine if you are even close to having the correct prop.Any numbers stamped on the hub of the prop you have now?
GOTCHA. I usually use 89, but I'll make sure to keep it at 87 or 89. Well I greatly appreciate your help. So in conclusion, if I found some extra money, do you think it would be benefitial after I change the plugs, to put a 13 1/4 x 15 prop on there thats new? Thats APPEARS to be the size thats on there now, correct? So would a new one of the same size without any scraped,bent,or nicked fins be worth trying?