Can't get any speed

W

whynot

New Member
Jan 27, 2010
1
Florida
Boat Info
1989 34' Sadan Bridge
Engines
twin 454 mercrusiers
Hello everyone, first time in the forum. Very new to this and I have a couple of questions. I know you all have many years of experience and maybe can help. I just bought a 1989 34' Sedan Bridge with twin 7.4's. Everything I read so far says I should be able to achieve +/- 30 mph at WOT. My motors top out at 4,000rpm Star. and 4,400 on the Port. My top speed is 20-21 mph at WOT. I pulled the props and I have 18x20's but the Prop shop scan told me they are actually down to 163/4 x 18. Both motors run great, no smoke, good compression and smooth idle. All running gear and motors are stock except for the carbs, both are new Edelbrock 4V. What can I check or do next. Short of putting out the spinnaker sail, what are other Sea Ray boats getting with this set up.

Thanks,

Why Not
:huh:
Miami, Florida
 
Bottoms clean?
Props clean?
Regardless, I'm not sure if a boat that size will ever reach 25 knots with 7.4s with that age. 4000 RPMs is a little low for WOT.
 
When we first got our boat it would do 22 knows with 1/2 full gas tank near empty water tank and empty holding tank. We added 10 six volt golf cart batteries, 2 more start batteries and a water maker. With all tanks full the boat will do 18 knots and RPM is 3800. I assume if we take all the extra weight out we could get the speed back. Point is you may be carring extra weight and that is slowing you down.
 
I have an '88 34' express. Top rpm is about 3600 (32 mph), with a cruise of 28 mph @ 3200. When I bought the boat (used) top RPM was 4400 with 24 mph top end. I compared with a similar boat in our marina and discovered I was underwheeled. E-Mail SeaRay and they will tell you what wheels you are supposed to have. Another thing could be that your wheels are flexing under load, make sure you have a good set of nibrals. Good luck.
 
Gaucho said:
I have an '88 34' express. Top rpm is about 3600 (32 mph), with a cruise of 28 mph @ 3200. When I bought the boat (used) top RPM was 4400 with 24 mph top end. I compared with a similar boat in our marina and discovered I was underwheeled. E-Mail SeaRay and they will tell you what wheels you are supposed to have. Another thing could be that your wheels are flexing under load, make sure you have a good set of nibrals. Good luck.
Click to expand...


Sea Ray told you to use wheels that only render a WOT of 3600 RPM.....?! I'm not positive, but those motors should turn a minimum of somewhere in the neighborhood of 42-4400 RPM. IMHO, I'd say you are now overpropped and overloading the motors.
 
I have a buddy with the same boat His WOT is 4400 RPM at 26 MPH He runs the boat at 3400RPM at about 18-20 MPH He has new props on it not sure of the size but I can find out ??????????????
 
whynot....you seem to be making/close to recommended WOT RPMs. I don't know what it should be, but your Top Speed of 21 MPH (18 knost) seems very low to me.

Two things....400 RPM diff between Starboard and Port motors is a lot....I assume that you are using the standard analog tachs to measure these WOT RPMs....they themselves may or may not be a part of that problem...you may want to verify those numbers with a laser tach. You may also want to confirm that you are getting full throttle via the throttle cables. Second...your speed....what are you using to measure it....GPS is prefered....not one of those transom/hull mounted speed wheels I hope?!
 
Something is definitely amiss. Here's what I've got: I turn 4200-4400 at WOT, the props are three blade 20x20c (cupped) and I consistently move along at a shade under 31mph at WOT, even with full fuel. You are either getting incorrect speed numbers OR you are propped wrong. SR original spec is 20x20c (from the parts manual). I'm not sure if incorrectly adjusted carbs could cause this problem. I'm guessing you have a problem with your wheels, although you WOT rpm isn't that far off.
 
Something's really wrong if he's running 17 x 18's and not getting to max rpms- and Ron's running 20 x 20's- and does...

Poster above asked about clean bottom and gear- have you confirmed this? I mean REALLY confirmed... nothing against divers, but I don't take anyone's word for it...got a wet suit, mask and snorkel? Trust me- all it takes is a few barnacles on your props- you are in FL after all- and you'll barely be able to plane. You MUST confirm your running gear and bottom are squeaky clean before you go any further on this witch hunt.

IF the bottom's good, and IF the engines are mechanically within specs- the only thing left is your hull...if you get to that point, have it weighed on the Travelift. You could have water trapped somewhere in the hull; it doesn't take much at 8 lbs./gal to add a bunch of weight to a hull.
 
Good point abt the fouled bottom. End of the 2008 season, the boat was dragging, and I mean dragging. Hauled out for the winter and found this mess, which cost me about 5mph off WOT speed. Not much growth but enough.

100_0902-1.jpg
 
Hey Ron....Did the yard wash the bottom after the haul? I'd be surprised if you had that much growth on the painted bottom that your props and metals weren't fouled also.
 
I never get any growth on the running gear. Not sure why. You know I get under the boat pretty frequenty during the season and I never needed to scrape. I think at the lowest point of the hull, the water may be a bit less fresh, both in Toms River and the Metedeconk, so stuff tends to grow there more robustly. Also, I think I figured out that I had completely forgotten to paint that entire section the spring before. DUH.

My previous boat had an outdrive and that would only get barnacles on the lower unit. The one spring I had that boat and I prepped and painted the bottom (the dealer did it the first season after I bought it), they had taken the clear antifouling outdrive paint off the market so I didn't spray the drive. Even with no antifoul, I had maybe 30 barnacles on that lower unit - that was at Green Cove Marina.
 
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