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Fuel Economy / Jack Plate?

7662 Views 33 Replies 12 Participants Last post by  BrionLutz
I love my new boat, but it has a drinking problem. It is a 19'6" Duckworth with an 18-degree hull and a Merc 150 2-stroke on an offshore bracket. The motor is a '96 Black Max XRI (fuel and oil injected) with a custom 5-bladed stainless power prop (not sure of the pitch). I haven't exactly been light on the throttle, but I'm only getting about one mile per gallon (seriously, maybe one and a half, or 22-25 gph burn rate on plane). I only have a 38 gallon tank, and some of the fishing I want to do exceeds my range (I don't mean tuna, I just want to be able to safely run around nearshore in the ocean or be able to run up to Kelly Point if the fishing is slow in St Helens, etc).

I am planning to add a fuel flow meter to check optimum trim angle and rpm, as well as to determine which future modifications may help reduce fuel burn (I am planning to add a "whale tale" or "edge" hydrofoil, and also to try other prop pitches). I am open to any suggestions regarding fuel economy, but one specific question would be if a "jack plate" might make a noticeable difference. I moore the boat, so I haven't had it on the trailer since the day after I bought it, but I'm wondering if the motor might be hung too low (the motor is a long-shaft, and it is mounted on the offshore bracket which is well below the transom height). I don't know anything about jack plates except that they move the motor back slightly and allow you to adjust the height for optimum performance. I really don't want the motor further back, but I would do it if adjusting the height would make a significant difference. How far below the transom should the cavitation plate be? With the motor 2 1/2' back on the offshore bracket (bracket angles up from the transom rather than having a planing surface), does it need to be below the transom at all? Also, I noticed that the more expensive jack plates are hydraulically adjustable while the less expensive ones are adjusted by loosening or tightening adjusting bolts; is there a huge advantage to adjusting it on the fly, or could I go the cheaper route and just leave it once it's adjusted correctly?
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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!
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Oh, one other thing-- I noticed you plan on installing a hydrofoil. If you're having trouble with your hole shot or with keeping the bow down at low planing speeds that can help, but otherwise it won't help your efficiency and might hurt by adding a little more drag. Proper use of the trim switch, the right prop, and the right motor height would be the first things to try before installing a foil.
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.
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When you say you are geting only one mile per gallon, I believe this is incorrect. The only way to measure gas consumption on water is with a GPH meter. Gas per hour consusumed. This could be many miles or less depending on water conditions.

With the GPH meter you can see the correct RPM to operate at in order to get the best distance out of a gallon of gas.
Install a GPH meter on your boat for accurate readings.

Good Luck;
If you know how many gallons you burn and have a GPS why can't you take the total mile vers the total gallons just like a car?
Another item I'd recommend is pulling the boat from the water and checking to ensure you don't have some marine growth "anchors" below the water line. It doesn't take a lot of marine growth to really increase underwater friction. Can you raise you engine one or two holes on the transom? That would be way cheaper than a jackplate. I'd also second the thought that a 150 should not be drinking that fast at WOT -- 15-17 gph would be about max, I'd think. Boat Doc would know.
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!
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Back off the gas when on step.

My plate does more good with a pump mileage wise than with the prop. Not enough difference to justify itself in a prop only situation.

Call Stevens about the prop pitch. They should be able to point you in a good direction.

Mark and the dog
Thanks DB. It has some growth, but not a lot yet. It has been in the water since I bought it in March, but the growth seems to get worse once the water warms. I have to pull it out of the water to do my 10hr. maintenance to the new kicker, so I'll look to see if I can move the motor up using the existing mounts.
"Back off the gas when on step."

What did he say, Dear? He said he thinks he knows you! :smile:

Starfish, thanks again for the input. I'll have to see if I can feel it losing it's grip; I've mostly just heard it. I'll let you know when I see what pitch the prop is. Also, thanks for the disclaimer; I didn't mean to assume that you are an expert, but you obviously have more knowledge than I do.
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.
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"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.
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I run a prop year round. One thing I have noticed is with the larger pitched props, you will burn more fuel. With a boat of your size and hp, I'd be running a 15-17 pitch at the most. The heavier the boat the smaller pitch. If you were running a bass boat then go to the 21+ pitch. Not knowing what you're running on your boat, but looking at the picture, i'd bet you're running a 20 or better.
Hmmm...at 1mpg with new boat and motor, you either have a serious problem or your calcs need some fine tuning.

Here's the Yamaha Performance Bulletin for a 19' Duckworth with a 150HP 4 stroke. 6.26mpg at 21mph is best. The worst is WOT at 3.16mpg. Two strokes use a bit more fuel but not 3 times the amount of fuel of a 4 stroke.

You probably need to check your calcs first. Something is way off.

Yamaha Performance Bulletin 150HP/Duckworkth 19' Advantage.
Look at "Floscan" GPH meters for accurate gas consumption readings.


Good Luck
I already have a FloScan but I'm not sure the FloScan would work on the older 96 Merc that Beefcake has...and they are expensive...mine was $800 not counting installation.

Something is really wrong with the original 1mpg estimate...the motor would have to be dumping fuel out the exhaust (which is possible if the engine is shot) or he's got the anchor out or something.

First thing is to figure out what is the real current mpg.
Rule of thumb says max WOT fuel consumption is hp divided by 10. In other words, your 150 should burn 15 gallons per hour at wide open throttle. At cruising speeds you should burn 8 gph. I suspect you have two problems. First, you are seriously stern heavy. Try temporarily taking 300 pounds out of the stern (gear, extra batteries, extra fuel, etc.) and see if things improve. Second, you may have the wrong prop pitch. The only way to check that is with a tach.
If you paid $800 dollars for a Floscan meter you are way above the normal cost.

You can check the going price on the internet for your engine.
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).
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