[lvg]

Our houseboat (Waterhouse 14' x 44') has 1996 75hp mercury 2 stroke. It is turning a 14" x 10" prop. But cannot reach my WOT @ ~5000rpm nor theoretical boat speed (8 knots). We are looking for the most effective powerplant/prop combination to have optimum fuel efficiency, has well as meet hull speed. I am considering increasing Hp to 90 or 115, 4 stroke. This unfortunately will not increase my propeller diameter just raw horsepower. Any thoughts on this would be very appreciated. Thanks in advance.

[JB]

Howdy, Perfectspot.This sounds like the kind of situation that Hi-thrust and Bigfoot lower ends were designed for.If you can't find a 14"x8" prop, look into a bigger engine designed for houseboat-big displacement hull applications.Good luck.

[LubeDude]

Because you are basically pushing a box through the water, You will be better off with the maximum HP the manufacturer will allow. This way you would be able to back off on the throtle a bit instead of maxed out all the time, even now, you would cut you fuel use cinsiderably by backing off just a little.

[BillP]

With that size boat I'd go for the 115 4 stroke. The extra ponnies may come in handy in bad sea or wind conditions. Normally I'm anti 4 stroke but fuel burn is a big factor here. Can you access the engine easily? 4 strokes need oil dipstick checks like cars do. For prop changes you may want to try 4 or 5 blades in flatter pitches to dial both power and rpms.

[lvg]

To Reply Authors,More horsepower seems to be the consistent theme since we cannot increase wheel diameter on our 75hp merc. It’s too bad the outboard manufactures do not make a 115 hp engine with the lower unit of a 225. That would give us propeller options of up to 16” diameter as well as a bigger gear case. I believe that’s how Mercury created the Bigfoot. By adding a 75hp lower unit to a 60hp power head. As for as diameter…would a bigger diameter prop. add more thrust? Or should we not even concern ourselves about that?

[BillP]

I wouldn't get hung up too much on an inch or two of prop diameter. The "traditional" mold has been broken somewhat in OB terms of bigger props being the only way to get max low speed thrust at high rpms. You can load the motor with blade surface to get max thrust without cavitation. Get a 4 or 5 blade prop with flatter pitch to match the motor's rpms and torque curve you want to run in. I'd do 5 blades but think finding one in a low pitch will be a problem. One of the issues with OBs on rigs like yours is they move surface water and lose fwd thrust. Using a jackplate to lower the motor deep might be an option to consider. Is that an aluminum pontoon boat? It's big for 75 ponies.

[lvg]

to: BILLPI thought of the 4 blade prop. and approached a couple of prop. manufactors about the 4 blades vs 3 blade effectiveness relative to my issues. What was suprising is that all of them had a different recommendation and swore by their recommendation. Actually, I have more faith in the replies I have read on this web site. Are you familiar with the prop guards? Some Manufacts. claim they add more thrust. Our boat hull is not pontoon. The hull is more like a cathedral hull or tri-hull. It tracks extermly well.

[BillP]

Props are as controversial as oil...Different shapes and sizes can produce the exact same results. No matter what the rpms or speed are: Traditional lore is larger diameter and less pitch is the power prop and smaller diameter with more pitch is a speed prop. It can be done with any number of blades running at the same rpm. Today it seems to be heading the other way. Years ago you couldn't find a raceboat or high speed flats boat with anything but a 2 blade prop. Now you see 5 blades dominating the F1 racing circuit and 4 blades dominating the flats boat crowd. They get both low end hole shots AND speed. Ships used to have 3 blades and modern ships have 4 or 5 blades. The trend is more blades no matter what the application. That's why I suggested more blades to harness the power without cavitation. It just gets expensive buying props to find the right one. The best alternative is to buy the largest diameter bronze 4 blade of the pitch that appears right and try it. If it doesn't work take it to a prop shop and have them tweak the diameter or pitch to meet rpms. Bronze is the easiest material to rework and most cost effective way to experiment. That is my standard practice on inboard boats and the performance changes have been exceptional compared to stock factory props.Good luck. Let us know how it works.BillP

[rickdb1boat]

It's a 44' boat with a 75HP on it. Nothing outside of more HP is gonna help. Go to the max. on HP and be happy. I turn a 14 X 11 prop on my toon with a 70 HP rude. Was using a 14 X 13 before. Something seems wrong with that setup you have. And I think that something is lack of HP. 150 HP would be where I would go. And yes, a larger diameter prop on a displacement hull will give you more thrust. Just ask the Merc. Bigfoot owners. What is the Max. rated HP for that rig?? Got to be alot...BTW- a 14 X 11 prop is suggested for a 25' boat according to the prop selector.

[rodbolt]

hello dude your pushing a barge. no amount of prop playing will help ya. a 75 aint gonna push a 14foot wide 44ftlongbarge. it aint gonna happen. I fought a customer of mine a few years ago on his 36 ft house pontoon. had all the amenities. hardtop shower, stove but he bought it with an elcheapo 60mariner. that was the smallest motor he could buy it with at the boat show. I argued with him the entire week. at the end of the show he bought it from a dealer in richmond va and had it delivered here.4th trip out he crashed it into the manteo bridge. that poor 60 at 5500 rpm could not deal with a 4 knot tide and a 20 knot northeast wind. the next week we bolted the 150 on.were it mine I would optfor the 4 stroke as they seem to tolerate lugging and low speed abuse better than 2 strokes. just make sure its at least 115horsepower. and FI most FI engines incorperate a knock sensor that detects detonation from lugging and corrects the ign timing and fuel delivery to eliminate it. good luck and keep posting

[BillP]

You're basically stuck with the outboard rpms but here are some general specs taken from Skene's Elements of Yacht Design. Somewhat general but better than nothing and can be dialed in closer with exact specs from your boat. Ad-libed with my comments.40~ waterline length max hull speed (displacement type) is 8 to 8.5 mph. More power won't make it faster in a displacing mode (any and all boats).50 hp is required to make max displacement speed for a HEAVY displacement boat (30,000-40,000lbs) Expect prop efficiency to be reduced by 45%-55% on barge types (this rule written for 3 blade props).Props WITHOUT foil shaped blades are most efficient for low speeds with high rpms. Cavitation is the reason.

[lvg]

To : rodboltHave you received any feedback from your customer regarding the large O/B he is now using?To: BillPIf “ prop efficiency to be reduced by 45%-55% on barge types (this rule written for 3 blade props)” then my search should focus on the 4 blade types. And if I’m understanding the last sentence “Props WITHOUT foil shaped blades are most efficient for low speeds with high rpms. Cavitation is the reason”, than I should find a prop with no foil shape. With out any further research, I think all OB props are foiled. There will be a challenge to find one not foiled.I started the hints to the wife about upgrading to more horsepower. A couple of more weeks and she’ll see it my way. I have a question about WOT on my 75hp mercury which I will post on the appropriate page. Thanks again.

[BillP]

Those design rules are just sound and proven principles to start with. In a practical sense it can't be calculated to the exact size. The best bet is to try several props. I'd probably start at pitching for 8 - 8.5 mph @ about 75% power and have 25% reserve power/extra rpms in winds and waves. Running on the slower side of hull speed will reap huge rewards in fuel consumption. 4 or 5 blades are more efficient at low water speeds. Flat blades don't churn as much air at high rpms as foiled blades do. Good luck finding a prop. You may end up with one of the old design 3 blade outboard type. Those blades have a lot of surface compared to modern props and little foil.

[Dhadley]

We use a 46' houseboat and its powered by 2 225 Yamahas. Sometimes 3. In any kind of wind you'll be glad you have twins (or triples).The 75 is OK, you just need more of them.

[Forktail]

By not reaching your max RPM, you're compounding the problem of just plain being underpowered. You may actually only be getting 60 HP at the lower rpm. You may want to investigate to see if you can change your gear set in that particular outboard. Since you're limited with propping, you may want to gear down to get the RPM up and increase thrust.Don't know if you can, but you may want to check with a custom shop, or here: http://www.sterndrive.info/mercury-o...lutch_dog.html

[waterinthefuel]

Originally posted by Dhadley:We use a 46' houseboat and its powered by 2 225 Yamahas. Sometimes 3. In any kind of wind you'll be glad you have twins (or triples).The 75 is OK, you just need more of them.

Sometimes 3? What, you keep a spare 225 kicker on the side and use it when necessary? The houseboat that guy said couldn't take a 20 knot wind? If the winds are 20knots don't take a houseboat out! It's supposed to be enjoyable, and if your engine limits your wind speed to say, 10 knots, then keep it that. 20 knots is almost 25mph. I don't know many houseboats that are very controllable in those conditions.

[Dhadley]

Actually the spare is in the middle. It had a 250 powerhead and it got sold so we're back to 2 for the moment.Unless the whole thing gets sold.........

[lvg]

To ForktailI considered the gear change, but I do not know a thing about appropriate ratios, etc. YouÂ’d think the outboard manufactures could give us the option to purchase a high-thrust configured lower unit, mated to a horsepower of my choice. YamahaÂ’s high-thrust lower units on the small outboards look like effective technology. Apply that technology to big HP and I think we would have a winner.Trawler type vessels have the ability to change the gear ration and prop to find the optimum performance to meet their needs. Why not outboards

[dilligafda]

"Trawler type vessels have the ability to change the gear ration and prop to find the optimum performance to meet their needs. Why not outboards?"One word, COST. The cost of developing such a system would not be cost effective compared to the number of engines used in the same manner as yours. Although, some of us remember when Merc had a prop that would change pitch, much like an automatic transmission in a car. The thing never did catch on all that well and was dropped a year or two later. Good luck in finding a solution to your issue!

[phatmanmike]

i used to own a 34 cris craft " aqua home 34" 34 footer, that origonally had twin 318 I/O making 220 hp each and would reach 12 knots. seeing as to how your floating fortress is 44 feet, yowsers, with only 75 hp.... hmmmmmid start looking at how much power it had stock and look at the newer ones and check their weight versus hp and see if you can find some usefull info from that , but by any standards, you are underpowered.......