twin inboard boat needs prop help

[spoolin01]

My mileage sucks! 3 gpnm. I've got a FloScan/GPS to tell me the bad news. I've got an '86 Cruisers Esprit 337 express cruiser with twin Crusader 454s with aftermarket Holley carbs (650 cfm). The props are 17x20 4 blade (repitched when I bought the boat), original size as best I can tell. They sit in half tunnels. Boat is listed as 11,000 lbs, probably closer to 13K. The motors are old, the carbs are old (but were professionally rebuilt 3 years ago). I run 2800-3300 RPM typically, ocean use. 3300 RPM is 13-16 knots, depending on fuel load (up to 300 gal). Right after the carbs were rebuilt, I'd get .5 to .6 nmpg. It's down now to .35 to .4. Clean or fuzzy bottom makes only a small difference. The motor RPMs are very unstable starting around 2800RPM. One or the other motor will race ahead and it takes quite a bit of throttle tinkering to get a balanced 3000-3400 RPM. I first thought this was ventilation, but the speed picks up considerably as this starts to happen. Top speed when low on gas is maybe 20 knots at 3600 RPM. I don't know if the low RPMs is the wrong prop pitch (manuf says that's the original pitch) or a linkage problem (haven't checked this yet). From what I can tell of published performance reports of other boats, for the motors, weight and hull type, I should be getting twice the mileage and close to another 10 knots top speed.<br />I've just bought new Holley 80552 650 cfm carbs. I'll check throttle linkage and top RPMs with the new carbs in place. If I need to massage the props at all, I'm wondering if anyone can suggest which way to go, diameter or pitch? There's close to 4" of clearance between the prop and tunnel (the tunnel is also not quite symmetric around the prop, I think there's close to an inch less clearance at the top than at the sides) Do you think the runaway RPM problem is ventilation/loss of 'bite'? This is my first big boat and I'm fishing around to understand how to read these signs. This thing is eating my wallet alive!!

 

[Spidybot]

Re: twin inboard boat needs prop help

Ouch. Moving 13000 lbs around on the water by two gasguzzling engines never really can be done cheap. Sounds like your carbs may not be working properly but also timing and other stuff on the engines? Some of the consumption may controlled by the way you use it. Surely it won't get the same mpg at all speeds?<br /><br />I do not believe in ventilation as cause for the revs. You say 'tunnel' which leads me to belive you are set up with fixed props rather than a i/o drive? If so I'd say ventilation is out of the question.<br /><br />A proper check of all settings and linkage most likely will show a number of errors.

 

[Stanp]

Re: twin inboard boat needs prop help

I have a 31' silverton with 350 crusaders (454's). I'd recommend reading up on props and such on www.yachtsurvey.com and look at the engines section. One way to check props is taken the engines to WOT. You should top out at b/w 4200 and 4500 rpm's. Also, the aftermarket 650 cfm carbs concerns me. I beleive OEM for these engines is 850 cfm Q-jets. The tunnel design you have is considered to be less efficient in some designers circles, but thats not that important. Is there any bad vibration when you bring her up to plaining speed? Do you have V-drives? if so, your shafts are out of alignment or your prop blades are bent. As far as your props, I run 17X19 3 blades, and your props at 17x20 4 blades don't seem to be the problem in my mind. It not unusual to have to tinker with the trottle to get the engines in sinc. you may want to buy an engine sincronizer to get accurate rpm's out of both engines. Also check with an authorized Crusader dealer to make sure you have the right carbs on them. 650 cfm is too lite. My boat is similar to yours and at 3000 rpms I get 16-17 knots.

 

[spoolin01]

Re: twin inboard boat needs prop help

Thanks for the replies, guys. It is a straight drive inboard. I have a FloScan 9000 which I use to synchronize the throttles. The thing is, I can move both throttles forward and come up to 2800RPM, then one motor, usually port, will race up to 3500RPM, in 2 seconds. I have to throttle way back on that side, then as it slows, try to nudge the throttle back up. Often it races ahead again, even though the throttle positions are equal. Eventually, I can get stable RPMs and synchronized throttles at 3000-3500RPM. Is this really commonplace? I wonder if it could be carburetor behavior, the vacuum secondaries opening, rather than a prop problem. I hadn't thought of that before...<br /><br />What sort of mileage do you get with your Silverton 31? Am I wrong to think 3 gpnm is a little unusual? A 100 mile range with 300 gallon tanks at 15 knots?

 

[spoolin01]

Re: twin inboard boat needs prop help

The FloScan has allowed me to monitor fuel consumption across RPM range. Consumption is a steep function of speed up to about 7 knots/1800RPM. Once I hit hull speed, the consumption really rises, but from 2500RPM up to 3500 RPM there is some change, but not much. It might go from 0.4 nmpg to 0.33 over that range.

 

[L Christopher]

Re: twin inboard boat needs prop help

Spoolin01:<br />My father has a 28 footer he keeps in the water and was having problems with runaway RPMs with one engine. Turns out it was a corrosion problem of one of the props. You may want to check that and the "Zincs"

 

[Stanp]

Re: twin inboard boat needs prop help

Ok, so the port engine surges or races from 2800 rpm to 3600rpm in 2 seconds all on its own. Now do you feel the added push in acceleration when this happens or does the boat hardy move faster or does it feel like its, I don't know,what's she doing? Secondary's opening up do give a bit of a kick. Are those carbs the right carbs for that engines? <br /><br />I get 1 mile per gallon Approx. I got 250 gal tank. I've been to the elephant trunk which is 65 miles off Cape May and back with 70 gallons to spare. when I am running, I keep her b/w 3000 and 3200 rpm. I could go faster but, like you, my engines are 20 years old with 1200 hrs on them. THey run great though, crusader puts out the top of the line gas engine in the boating industry. Once again, go to www.yachtsurvey.com , I promise you that your knowlege of boats will double just by reading Dave Pascoe's articles on engines. <br /><br />Now, I may be way off, but is the trans slipping out of gear causing the rpm's to rise? Ok, next, Points, are they gapped properly. Many back yard mechanics have no clue how to set points. The advance - vacuum advance in the distributor advances the ignition of spark as the engine increases rpm's. if the vacuum diaphram is no good, no advance, no fuel effient burning. The coil, if the engine backfires as your increasing rpms, the coil's no good. Also, I've seen guys take the boat up to speed as another guy spins the distributor to get an engine timed properly. check to see the bolt holding the distributor is secured. Its got to be timimg in my mind. <br /><br />Buddy, we're going to get this, i know it! boy it just bothers the heck out of me why its doing that surging thing.