sea ray prop for planing

[jfc52]

Bought my first boat this summer. It is a 1979 Sea ray 26' express cruiser with a 350 (260 hp) motor and an OMC outdrive. It also has trim tabs. The boat would not get up on anything resembling plane. I realize this is not a true planing hull but we were just wallowing around. This past weekend we did some experimenting with props. The prop on the boat was an aluminum Michigan prop 15.5x13 and we got 17.9 mph at a max of 4000 RPM. I put the spare prop on and went out again. It is a stainless Michigan 15.5x14. We got a speed of 24.5 mph at max 4000 RPM. The local prop shop recomended a 16x14 Aluminum prop for best performance. Now the questions. The boat should operate between 4200 and 4600 RPM. What are the possible problems (we had the carb rebuilt a month ago)? What would be my best bet for a prop? We want to cruise but not tow skiers or anything. What should be my expected top speed for this boat?
Thanks
Jeff C

[Corpus Skipper]

We got a speed of 24.5 mph at max 4000 RPM
Was this wide open throttle? The best prop for your boat is one that'll allow the engine to turn 4600 RPM with an average load of people, fuel, etc.... Maybe even 4700 RPM to allow for heavier loads, bad conditions,.... At 24.5 MPH the boat should have been planing nicely. Play with the trim tabs for the best speed, too much tab will cause the bow to plow and slow you down, not enough will cause the stern to drag, with the same effect. Between trimming the drive and tabs, you should have no problem finding the "sweet spot". Having trim indicators really helps to dial in the best combination, and makes it repeatable as well, without a lot of fiddling.

[jfc52]

This was throttle wide open both times. I had the trim tabs down and there seemed to be no difference if they were down or up. I don't have a tab indicator but I know they are working. At 24.5 mph, the boat was almost on plane but just not quite. If I could get more rpm I think it would work. Any suggestions as to what prop I should try or how to get the rpm up?

[Corpus Skipper]

I would go with the prop shop's recommendation. I stand by the fact that the boat was on plane at 24 mph. She runs bow high I suppose is why you feel she's not. Try shifting some weight foreward to bring the bow down. Watch your wake at speed. It should be a long, narrow V at 24 MPH. At transitional speeds, the wake will be wide, high, with a short, pronounced rooster tail going almost straight up, and the boat will leave rollers behind her, moving in the same direction of boat travel. If the trim tabs are ineffective, you may need larger ones, though a well balanced boat doesn't need them at all, except for leveling from side to side to compensate for passengers moving around, etc..

[yipster]

24 mph must be in plane i second. sure props are important but i sense your trimming isnt rite. even minor adjustments in smal tabs should be clearly felt and once aquanted you dont really need the indicators. did you try trimming the sterndrive a little up and down, you could be nose down also. play with it some more before changing props, and than and if, go for stainless hi-five better than alu.

[Willallison]

The maximum displacement sdpeed for your boat is between 6 & 7 knots - say 8 mph. Any faster than that and you begin the transition to planing mode. So, as the lads have pointed out - at 24 mph you are definitely on the plane.
I had a 27ft Searay Sundancer with a 260hp Mercruiser - essentially the same engine and boat as yours. Flat out it would pull around 37mph. Unfortunately I can't remember the prop size - but your boat should manage roughly that speed too.
When was the boat last antifouled - weed on the bottom can soon knock 10 mph off your top speed - and add mightily to your fuel bills!

[jfc52]

I appreciate all the input. How do I achieve the legendary 37 mph? What is the concensus on the fins you attach to the cavitation plate (doelfin)? Do they help? How can I adjust the load? Water tank, fuel tank etc are pretty much stuck where they are. About all I can move is my tool box. A friend was telling me that a lot of boats of this class are running props with a pitch of 20 or more. Would a prop of about 17 help?
Thanks
Jeff

[Corpus Skipper]

If you're only getting 4000 rpm max with 14 pitch, going up in pitch won't do anything but overload the engine. Go with the prop shop's recommendation, they know their stuff. If you still have the need for speed, put your boat on a diet. A friend removed over 100 lb of miscellaneous "stuff" from his boat, towels, 10 half used sunscreen bottles, old parts (as "spares"), a few pumps that were replaced, but the old ones left in because they were dificult to get at, sinkers, about 20 lb of nuts, bolts, clamps...... you get the idea. :D

[Willallison]

Corpus is right - going up in pitch doesn't sound like the solution to your particular problem. I think I ran a 17" generally. I had a 19" but when the boat was loaded for cruising it was just a tad too much.
To give you some idea I used to cruise at 3500 rpm and about 20 - 22 mph.
Also, whilst I agree that pulling weight out of the boat is a good idea, unless you are hiding the US olympic team down there, I doubt whether you'll pick up more than a couple of miles per hour.
I had fins on the cav plate. They may or may not make a small difference in some areas - but they certainly won't give you another 10mph.
You didn't answer the question as far as the boats bottom goes - is it clean?

[gonzo]

If you only get 4000RPM, the engine is overloaded. This is assuming the engine is in good shape and tuned up. Overloading an engine produces excessive combustion chamber pressure and heat. It will, sooner or later, blow holes in the piston tops. Also, it wears bearings and rings faster. The pitch is too high if you don't reach at least 4300 RPM.

[jfc52]

Thanks again for all the advice. The bottom is almost clean. I realize that can make a difference as the season goes on. I am wondering now why I am only getting 4000 rpm? Will remove or reposition the dead weight (tool box, anchor, etc) and see if that helps.
Jeff

[woodboat]

Do you know if the engine is making full power? Did you run a compression check? What was the original prop supplied with the boat and RPM achieved as delivered from the factory?

[jfc52]

The boat recently had a rebuilt carb installed. Did not run a compressioncheck but it seems to run very well. With both props mentioned I was only able to get 400 rpm at wide open throttle. Don't know about prop or rpm from factory since I just bought it this summer. The book says it should operate from 4200-4600 rpm.

[firestorm]

Sounds to me like you are getting about 180 hp out of the motor OMC prop chart for that engine and drive and you boat size and type shows 15.5x14 prop at 4600 rpm and 27-34 mph. The fact that the engine starts and runs ok does not mean it is at full strength just that all the cylinders are worn evenly and firing together. Compression should be in the 125-135 psi range per cylinder. But will start and run as low as 90 psi and still sound decent if all of them are equal it will have a smooth idle and run just not much power. If you can borrow a prop for testing my bet is that a 15.5x13 will put you in the correct rpm range leading you back to the low power issue. If you can't get full rpm with the prop you have than going to a higher pitch is the wrong way. Taking a SWAG (sientific wild ass guess) it sounds to me like you are somewhere between a 4bbl V-6 and a 2bbl 5.0 on power. Good luck and get the engine checked out.

[firestorm]

Something I forgot to mention in my earlier post is that if the compression checks out ok check the camshaft as GM had a huge problem with camshafts in the late 70's and early 80's excessive wear caused very low power even though they still ran fairly well. Not being able to breath may be the problem as when the cam wears it no longer opens and closes the valves correctly causing low power and loss of rpm. This should not apply to a marine engine but unless you are the original owner you can't be sure it has the original engine. You can check the cam with a degree wheel and a dial indicater it is a little time comsuming but easy to do put a degree wheel on the balancer and mark it remove the valve covers and set the dial indicater to measure the pushrod side of the rocker arm, rotate the engine by hand untill the valve is fully closed set the indicater and rotate the engine untill the valve starts to open and make a note of the degree wheel mark, rotate untill the valve is fully open and make a note of how much the rocker traveled with the dial indicater this will give you a rough lift figure, continue rotating the engine till the valve is fully closed and note the timing on the degree wheel. the number of degrees between valve begining to open and closing is your duration and the movement of the pushrod side of the rocker is your lift. If you want to get real technical put a second dial ind on the other valve for the cylinder you are checking and note the degree wheel markings that both valves are open this is your valve overlap. I do not have the exact specs for your cam but most of the ones I have seen run about .500 lift and in the 260-280 degree range with a 280 degree cam you would have a slight lope or rumble at idle but great power this is near what they used to call a 3/4 race cam spec. One final note with the valve fully open roll the engine back and forth and watch the valves this will give you a idea of how much slop is in the timing chain and gears. if you have more than a degree or so of slop you need to change the timing chain and gears as this will also cut your preformance since the actual timing will be constantlly changing. Hope this helps.

[jfc52]

Thanks Firestorm and others for all the help and suggestions. Will check the compression and carb to make sure the other two barrels are opening. Sounds like the prop shop recommendation is about right for the prop. Need to find out why the rpm's are low.
Thanks again.
Jeff

[jfc52]

Thanks Firestorm and all others for all the info and suggestions. Will check compression and carb to see if other two barrels are opening. Sounds like the prop shop recommendation is about right and maybe now it is the motor not getting the rpm's. Will keep you posted what happens.
Thanks
Jeff

[jfc52]

Checked the motor this past weekend. Compression is 130-135 on all cylinders. The four barrel is opening up properly. The motor revs fine in neutral. Am I overpropped? Should I go to a 14" x something to get the rpm's up?
Jeff

[firestorm]

Glad to hear compression and carb are ok You still are not making full rated power if you were your rpms would be right I still wonder about the cam wear and also if it is a true marine engine not a car engine with marine parts on it. I am shooting from the hip here but a car engine would give you about 165 to 180 hp which I think would land you about where you are at on rpms with the 15x14 prop other possibability is a marine engine rebuilt with automotive parts cam ect... running the wrong cam would drop your power quite a bit and kill your rpm as the power curve for a chevy is not where near where you want a marine engine. THe average 4 door gm is planned to run about 160 to 200 hp at about 2500 - 3500 rpms for a 100,000 miles and seldom ever goes above 4000 rpm many were geared to run even lower rpms in high gear so the cam is not ground to give high lift or duration but is set to give best pulling power at the 2500 - 3500 rpm range. one other thing to think about is if the carb is jetted to run the rpms you need you may check with the people who built the carb and see what it is set for it should be jetted for at least 600 cfm. A simple carb formula is cubic inch displacement x desired rpm / 3450 plus 10% ie 350cuin x 5200 rpm = 1820000/3450=527.536 cfm +10%= 580 cfm carb. Also Mr Gasket offers a program called desktop dyno that is suprizingly accurate and it allows you to build mock engines and dyno test them we ran a mock up of my brother in laws 383 small block before he took it to the dyno and came up with 807 hp it dynoed at 812 so the program is pretty close to real numbers. However it cannot take wrong parts info and poor tunning into consideration it uses ideal correct settings. Hope you get it figured out good luck.

[jfc52]

Thanks again for the info and advice. I am convinced the motor is a marine engine and is running as well as it can. Would going to a smaller prop (14") give me higher rpm's? Would I go to a 14x15 or higher pitch? Would a higher pitch(17) negate the advantage of smaller diameter? What I really need is a try it before you buy it program.
Jeff

[firestorm]

OK I still beleive power can be improved but that can come later try this to get a prop recomendation. http://www.ballisticprops.com/MIWheel/propit/main.htm
This is a online prop calculator and if you give them the correct info they will give you the right prop for the job.
It is not the fastest service in the world may take them a week or so to get back with a answer but you will get one. Most of my posts have centered on the power issue as a whole it takes a certain amount of power to move a boat thru water at speed. In rereading your replys to me and others I noticed that in the 2 props you have tried the rpms stayed the same only the speed changed this again makes me wonder about the engine cam and possably the igniction system as a curiousity what does the engine free rev to in neutral? If by some strange deal the engine has been limited in rpm due to a limiter or by malfunction the you may be able to go to a larger pitch prop and gain more speed. I say this because usually a minor prop change has more effect on the full throttle rpm than the boat speed and you adjust prop size to get enough load on the engine to keep rpms to a reasonable level. Simply put a smaller prop on most setups raises engine rpm but boat speed stays about the same just has faster acceleration and runs higher rpm at top speed. A higher pitch prop normally does the oppisite boat is slower to accellerate and engine labors at a lower rpm to make speed. A friend of mine had a master craft ski boat that had a rev limiter to allow them to use a lower pitch prop to get out of the hole and limited engine rpm to prevent over reving. I wish I had kept the 14.5x19 prop I had cause I would loan it to you to try but I threw it away a few years ago when I decided I was not very interested in boating at the time. go try the calculator I gave you and see what they say.
Good Luck