1986 268 weekender engine speed and planning?

L

lime4x4

Member
Jul 24, 2007
341
Palmerton,Pa
Boat Info
1986 268 searay weekender
Engines
5.7 vortec alpha
We have a 1986 268 searay weekender with a 350 mercruiser engine.In order for this boat to stay on plane i have to run the engine at 4000 rpm's which maintains a speed of 27 mph.Current prop is a 15.25X15 3 blade alum. Is it normal to have to run the engine that hard just to maintain plane? If i try to knock the rpm's down to 3800 rpm's the bow starts to come up and she starts to go off plane
 
Bottom paint ? Generator ? Trim tabs down ? tachometer accurate ? You have the prop that sea ray specified :
1986
268 WE
260 Merc
1.5 gear ratio
15.25 X 15 prop
4600 WOT

Does seem like you are having to push it hard. I had the same motor in my 24' sundancer and would plane at 2600 rpm and 24mph. Although it was a smaller boat.
 
tach is accurate. The boat maxes out at 4300 rpm's. Trim tabs are fully down and no generator. max speed is 34 mph
 
Something doesn't seem right. That is a lot of boat for a 350 and it's working pretty hard. But a 300 rpm change isn't going to push the boat from 27 mph to 34 mph. It should be on a good plane at 27 and that small incremental change in rpm's isn't going to add 7 mph hour either. I'm guessing bad tach or spun hub. Has this problem existed for a while or did it just surface? Do you smell burnt rubber by any chance? There's not really enough information at this point to even guess! Is the speed verified by a gps?
 
I am very familiar with 260's and 268's of this vintage. With 350's in real world situations, they usually do require 3800 or more to stay cleanly on plane. My 260 came with a 350. I changed that powerplant, in large part, because of poor performance.
 
i will test mph this weekend with gps.The boat has always been like this since i bought it.Currently has 12X12 bennett trim tabs. According to bennetts site i should be using 30 X 9 trim tabs.The boat probably weighs reall close to 7500 lbs when fully loaded with gear and fuel.I was thinking of putting a big block in but wasn't to keen to adding more weight to an already heavy boat
 
268's really aren't very heavy (around 6,000 pounds) and they like big blocks. I think you would be surprised at the performance difference with a big block and Bravo III. It really adds a great dimension to an already exceprional boat.
 
well when i had the boat and trailer weighed the boat weighed in at just over 7000lbs. The boat isn't water logged.That was with the fresh water tank filled and 3/4 tank of fuel and msic gear. I would love to install a big block and bravo III just can't justify the almost 10 grand bill to have it done. Would love to find another searay weekender of that size or slightly bigger with a bigblock or even dual small blocks
 
I have a 1980 260 Weekender with a 350 and Alpha drive. Yours may be just a couple of inches wider and a little flatter - but not totally different.
At 27 mph you're not starting to come off plane by cutting 200 rpms. Mine will stay on plane at 2000 rpm and half that speed. Maybe the bow coming up means the boat is returning to its intended angle. Maybe the angle of your outdrive and your use of tabs is pushing the bow down - which may also be slowing your speed. Try reducing your tabs, and try raising your outdrive just a little. If you raise it more than a little, you will have trouble controlling the boat. Finally, look into a stainless prop. The aluminum ones just aren't strong enough to take the full force of a 350. The Merc Mirage 15 may work well. You won't get a whole lot more top speed but you should be able to cruise at 3000-3500.
 
okay things to check next time out. I'm use to when i'm up on plane the boat is level. As i try to back off the rpm's the speed drops and the bow starts to come up. Our other boat was a 19 footer with a 4 cyl mercruiser/sterndrive setup. When i acclerated the bow would lift up then level off once on plane then i would ease back on the throttle and trim the outdrive slightly and the boat would remain level. On the 268 the trim tabs are fully down to help get it on plane and i usally keep them down cause they seem to keep the boat level but at the sametime i guess they could be creating a drag with less engine rpm's. I don't really care about top speed just would like her to stay on plane at a lower rpm range and at the sametime get a little better gas mileage without keeping the engine at 4000 rpm's. Were planning on going tomorrow night with the boat. I will try once on plane retracting the trim tabs and c what happens. I'll also have my gps to accurate mph. Also on the weight of the boat i also have 6 batteries. 1 is next to the engine for starting the other 5 are deep cycle located in the midberth storage compartment. The boat other wise is great plenty of storage, room to fish and is comfortable enough for the occ overnight
 
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Why do you need so many house batteries if you only occasionally overnight? You could shave off a few hundred pounds by dropping to say three group 27 deep cycle units. I think you should try bringing your tabs up and trim out after you get on plane then try to throttle back to say 3500. The bow will climb a bit, but that doesn't mean you are totally off plane. My '82 260 SXL with a single 350 OMC swinging a 15x16 SS prop will hold plane down to about 2600 with tabs. I usually cruise at 3000 at about 24 mph. Sounds like you have a slipping hub or a great deal of prop slip. Have you tried switching to another prop for a comparison?
Let us know.
 
Are you sure this isn't an electric boat?
 
I use that many batts cause when were out for the day i have the radio going,fridge going the kids have a portable tv and dvd player and sometimes when were on the lake we might stay out till 1 am fishing so the lights are then at night.It's a new hub and prop. I've tried other props and the 15.25X16 gives the best performance
 
also i've tried running running the boat with only 1 battery with the same results
 
Ok, for one more comparison -
I've got a similar but slightly shorter & lighter hull, same power & drive,
16 x 16 cupped aluminum prop.
I accelerate with tabs competely retracted and drive trimmed all the way down, once the boat is up on plane (perhaps 7-12 seconds depending on load) I bump the drive up a few degrees, throttle back and add a little port tab (right switch) to even out the roll, which I think is largely due to my own mass on the helm, which rolls it a bit to starboard. I can stay on plane down to about 2800 rpms, depending on the chop - and pull 4800 at WOT.

I really think this is a tab issue. I can't imagine running my craft with tabs fully extended.
 
kbsunlovers said:
I really think this is a tab issue. I can't imagine running my craft with tabs fully extended.
Click to expand...

Ditto. Stock prop. Tabs retracted, unit down. Once on plane I back off the throttle and play with the tabs to even things out.
 
okay finally got the boat out tonight on the lake.A little choppy but not bad. The boat got up on plane the boat maxed out at 4400 rpm's 29 mph with gps with tabs retracted. raised the outdrive up a little. Knocked the throttle back down to 3500 rpms.the speed decreased to 13 mph using gps. I'm going to try playing with the trim tabs a little and c if that helps.Didn't have much time tonight to really play. Getting ready to take the boat down to the upper chesapeake bay area saturday into sunday
 
Just wanted to add a bit but it looks like you guys have most of it covered I have had a few searays and I am currently running a 84 270 with a single 454, similar hull and weight distribution design just 2 ft longer and 2 ft wider than the 268

First I would suggest to anyone expecially with gas the way it is all over the country (here on Lake St.Clair it is 4.40 a gal) anytime running acrost the water unless it is very rough or there is a specific reason you need a very flat ride you should run with the trim tabs in the full up position and the drive as high as possable with out causing cavitation, this is usually about half what the trim will allow you to do, the main reason the tabs are on your boat is to add to the over all planing serface of the hull and to be adjustable allowing the operator to correct for improper listing or rough conditions by pulling the tabs down and forcing the bow into the waves or individually adjusting and correcting any listing the boat might be doing, keep in mind you are basicly putting a wall down in to the water surface and greating a great amount of drag, ( you also mentioned the you have 12x12 instead of 30x9 this could be why your boat is effected so greatly you have a additional 3 inchs of drag per side with less surface area)

Now once you have the tabs figured out you want to also adjust the drive up so the drive and prop and running through and pushing the water on the same plane as the bottom of the boat, when left all the way down your drive is deeper in the water (creating more drag) and it is pushing the water in a downward flow making it alot less efficient at moving the boat foward with out trying to lift the rear of the boat. Alot of people aren't sure or are scared to use the drive like this but trust it is 100 percent designed to do this, there are many reasons that you have the u-joints in the drives and this is one of the top ones, if you are worried about it at all crawl under any 4x4 truck and take a look at the front and rear drive shafts very similar u-joints are much greater angles that can last up to 100,000 miles all while taking on constant adjustment from the bumps in the road.
 
you might want to confirm your tabs a working. on that age of boat there could be any number of small prooblems. if you are in fact running with them down you are driving your boat off plane its not falling off.... as stated above
 
trim tabs are working. When we got the boat i installed the bennett trim tab indicators
 
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