Fuel Economy / Jack Plate?

Wow! Mine drinks fuel like that but it's a 225hp Merc and I run it hard. I would have expected 15-17 gph with a 150. Anyway, improving your boat setup won't decrease the number of gallons per hour your motor will drink at WOT but it will sure increase your top speed, so the net effect will be more efficiency.

With the motor on an offshore bracket you ought to be able to raise the motor pretty high without needing a jackplate. (An additional setback probably would not be desirable if you're already 2.5' back from the transom.) As the motor gets farther and farther back from the transom it can be raised more and more-- that's why the bass boats use setback jackplates, to let them raise the motors and cut down on drag in the water. My boat (without a jackplate but with a pretty aggressive step pad built in) has the motor set with the main cav plate about 3" above the bottom of the hull, so the top of the prop is just about even with the hull. That's an extremely high setting and the boat shop was skeptical when I requested it, but as it turned out it works fine because of the hull design (pretty unique for an aluminum hull).

Here's a picture for comparison:


The High Five prop you're running is a very capable prop for running in elevated positions. You should try raising the motor as high as it will go in the existing mounting holes and see how the boat performs. If it still doesn't give you enough height you could try a manual jackplate with the least amount of setback you can find, probably 3 or 4 inches. Keep raising the motor until you lose your grip on turns, then back it down a notch. If you run in a lot of really rough water you should err on the low side. If you're raising it very high keep an eye on your telltale stream to make sure the motor is getting enough cooling water.

The other thing you ought to check is to make sure you're running the right pitch prop. Check your WOT RPM's against the manufacturer's recommendation and make sure you're at the high end of that range when the boat is lightly loaded. But don't swap props until you've raised the motor-- raising the motor will raise your RPMs (and top speed) unless you've raised it too far. You can also try different prop designs; the High Five is an awesome prop for hole shot and for holding in rough water, but it has almost no reverse thrust and isn't the fastest top end (although not bad at all). I like the Tempest Plus (3 blade) a lot, best top speed and really good all around performance. Some folks prefer four-blade props such as a Trophy or Rev 4 for rough water use.

Good luck, let us know how it works out!

Wow, thanks Starfish. I'll need to chew on your response for a while, but that's a lot of good info. You hit the nail on the head with your description of my prop characteristics (great hole-shot but no reverse thrust - I got myself in trouble the first time I needed reverse badly).

The reason I'm considering a hydrofoil is too get more positive feedback from my trim settings. My last two boats were Merc I/O 's with Stingray hydrofoils, and there was much more response from a touch of the trim button. On this boat, I can't really feel much difference between full-down and trimmed correctly, and I usually end up trimming too high and porpoising (I'm not sure if this has anything to do with the offshore bracket or if it is just due to the lack of a hydrofoil and the light bow). By the way, do you have any opinion between the "whale tail" and the "edge" foils?

As I mentioned, I haven't had the boat out of the water, so I can't picture how high the motor is, but your picture gives me something to compare to as the upper extreme. I may email you a pic of mine next time it's on the trailer for your opinion. Your description of the advantages of raising the motor make sense, but I'm a little nervous based on your statement that raising it too high could make it skid in turns. It already skids more than I'm used to in tight turns (not badly, just more than I'm used to). I was assuming that the skidding was due to the motor being so far back - putting the pivot point further from the boat and having the torque from the motor pushing it like a fulcrum.

I also may have more questions for you regarding the prop. I have the stock prop that I can try for comparison, and I have a Merc 4-bladed aluminum prop off my old boat if it will fit (I'll call the dealer before trying it).

I'll try not to bug you until I have more information. I need to install a tach and a fuel-flow sensor so that I have a starting point to compare any changes to (for some reason it was apparently ordered without a guage package - no tach, speedo, voltmeter or anything, just a stupid inaccurate fuel guage). Thanks again for your help, and I will be back in touch once I have more pertinent information. I love the boat, but I need to get it dialed in for my uses.

I'd definitely get a tach right away before making any adjustments-- it's super important for the health of your motor to make sure you're running in the right RPM range. It'll also help you dial in the most efficient setup.

When I talked about losing your grip in turns, I didn't mean the whole boat would slide-- rather that the motor will ventilate and the prop will lose its grip while powering through a turn (you'll hear the motor rev suddenly) if it's set up too high. Same thing in rough water.

I don't have much experience with hydrofoils on larger boats so I can't make a recommendation. I did have a buddy with an I/O runabout and addition of a hydrofoil made a huge improvement in hole shot for skiing, but it doesn't sound like you're having trouble with holeshot. Trying to get more response from your trim setting can just as likely be fixed with prop selection or motor height, so I wouldn't rush into getting the foil until you've eliminated other factors.

Let us know what pitches your props are, especially the High Five, and when you get a tach so you can know your WOT RPMs you'll know a lot more about prop options.

One last caution-- don't assume too much expertise on my part! I'm just a fisherman with a need for speed, so I've done a little homework looking into performance issues on my own boats and have set up and helped tune a few of my friends. I'm not a professional... my advice is freely given and worth every penny!

Your cavitation plate should be a inch above the keel for each foot of setback. Use a straight edge off the keel With a vee hull you can go just a bit higher.

You may be able to move the motor up a couple of bolt holes with out making more. Bass boats use jackplates to lift the motor higher for speed, just the bottom of the prop contacting the water, but you get cavitation and you can 'blow out' at speed, which is a quick uncontrolled 180 type manuever. Also low water cooling water pickups may be required.

Fewer blade are more efficient, but one bladed props are kinda rough. I would recommend a 3 bladed aluminum spare prop as a spare. Something around a 19 inch pitch, go with a slightly larger than stock diameter if you want optimum cruising efficiency. Aluminum tends to break before your lower end when you hit some of that stuff in the water. Stainless blades are thinnner, so more efficient than aluminum.

The prop shop east of Oregon City will let you try their reconditioned props before you buy.

Is the bottom of your boat clean(them weeds slow ya down big time.) and is the bottom flat? a hooked hull suck gas too.

If you aren't chaseing bass (just how fast are those fish anyway?) you just need to get it adjusted once.

Having your motor 'to low' is good if you need to manuever at low speed in current and swell, like if you are surveying the jetties or some such.

You can break your cavitation plate with those wings if you are not careful.

What's a custom prop?? Don't prop manufacturers know what they are doing? Seem's like an outfit like Michigan wheel could build a prop after practicing a hundred years.

Does your weight distribution (your boat's) cause good trim? If not maybe trim tabs are in order or move cargo around.

Jack plates will increase you setback even more. Maybe (probably) you don't want that.

I think I saw some used 4 strokes in the classified ads, if you can slide that by your finacial officer that will give you better range.

Is your engine well tuned and not been modified, say with larger gas jets?

If you look where your motor is bolted on you will see that there are several holes to allow you to adjust the motor's running heigth, start there.

You should be around 2.5-3 mpg on flat water at 2/3 throttle it seems like.

Do you have a tach to check your WOT RPM?

Now you know why you see me buzzing around in the channel in my itty bitty 15gpm boat instead of one of my bigger ones.

"You can break your cavitation plate with those wings if you are not careful."

That's why I was looking at the two models I mentioned. Apparently they both attach below the cav plate rather than on top, which supposedly reduces the chance of breaking the plate.


"Does your weight distribution (your boat's) cause good trim? If not maybe trim tabs are in order or move cargo around."

Boat seems very heavy in the stern. I don't know how to lighten the stern, so I was going to try adding weight to the bow.


"Is your engine well tuned and not been modified, say with larger gas jets?"

Good question. I was told the original owner liked to go fast, so I'll have to ask him if he re-jetted it.


"I think I saw some used 4 strokes in the classified ads, if you can slide that by your finacial officer that will give you better range."

If I don't figure out how to get the current configuration more efficient, then a 4-stroke will pay for itself.

Thanks everyone. I will update once I have more info. After re-thinking everything, I think I made two wrong conclusions. First, I think I was a little heavier on the throttle than I gave myself credit for. I went for a ride today, and learned that the boat handles fine at 27-30 mph instead of 40+ like I had been running it; I just don't get there as fast. Second, I may have been off on my mpg, but not by as much as you might guess. I think I wen't further than I thought, averaging 75-90% throttle, so now I'm thinking I was actually getting about 1.5-1.9 mpg. Heck, if I back off to 2/3 throttle, maybe I can break 3 mpg.

I will be adding a tach and a fuel flow meter, so I guess we shouldn't speculate further until I have hard data. Thanks again for all of the input so far. I will be looking for more advice once I have the facts. I'd love to be able to run out for halibut someday (with enough fuel to return).