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Thread: Is there a difference behind HP - Gas and Desiel

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    Default Is there a difference behind HP - Gas and Desiel

    I am a little slow so please have someone explain. Is there a difference of HP in a gas engine versis a desiel. So my question is really if I have a 350 HP gas motor and it makes my boat cruise at 22 knots (the boat is a 28x10') will a 250 hp desiel have better performance? I understand the fuel economy will be different in fuel economy but what about cruise speed? If I should replace my gas motor should I look for a desiel with 350 hp or greater?

    Thanks in advance.

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    Member c-bolt's Avatar
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    The biggest advantages to diesels are the massive amounts of torque they make at low rpm, plus they are usually more efficient. There are downsides though, like weight and cost. Some say they are safer, others say they are the same...

    There quite a few boat builders putting diesels in the boats now though. I can only assume they would last a lifetime.
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    You can get away with almost 1/2 the horse power with a diesel . If your boat took a 350 HP Gas motor a 175/200HP Diesel should do about the same .

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    Quote Originally Posted by Rod123 View Post
    I am a little slow so please have someone explain. Is there a difference of HP in a gas engine versis a desiel. So my question is really if I have a 350 HP gas motor and it makes my boat cruise at 22 knots (the boat is a 28x10') will a 250 hp desiel have better performance? I understand the fuel economy will be different in fuel economy but what about cruise speed? If I should replace my gas motor should I look for a desiel with 350 hp or greater?

    Thanks in advance.
    The biggest difference is hp at the shaft at a specific rpm. Diesels have more hp and torque at lower rpms. Todays lightweight high revving diesels will perform equal to a higher hp rated gasser. As an example, the 250hp FNM will turn a hamilton 2.4 turbo impeller up to 4150 rpm which is equal to what a 330hp V-8 turns the pump. The pump is actually stalling the gasser at 4200 and absorbing ~250hp just like the diesel. In reality, the rating on the engine is only as good as the rpms it can turn the impeller. You could get 350hp out of the gasser if you decreased the impeller pitch but you would have to run very high rpms to see any performance out of the pump.

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    Diesels rule in just about any size boat with much better fuel economy. The only really big negative is their much higher initial cost. If you were not a charter boat it would take many years of better fuel economy to make them pay for themself. But the bigger the boat, the quicker the pay off.
    Tennessee

  6. #6

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    The relationship between HP and Torque is: HP = (Torque x RPM) / 5252, torque is in ft-lbs. All engine specs (diesel and gas) will be quoted by some HP at some RPM amount. So, it's not exactly an apples to apples comparison between diesel and gas as the quoted power will be at different RPM's. When you look at torque vs RPM on chart, the shape is considerably different between a gas and diesel engine. The diesel's torque peaks out at a much lower RPM than gas engines. So, depending on the engines, you'll be able to make the same HP in a diesel at a lower RPM. The diesels accomplish this because of the very high compression ratios (relative to a gas engine) used to detonate the fuel.

    I assume in your example that you probably make that 22 knts turning 3200 to 3500 rpm on your gas engine. With the proper gear ratio, a diesel engine may move your boat at the same speed around 2500 rpm's. This is just a guess based on running RPM's on boats I've been on.

    If I have completely confused the issue, then go read this guy's article. He hits on more topics than just fuel efficiecy when trying to decide between diesel and gas power.
    http://www.yachtsurvey.com/GasNdiesel.htm

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    Cizmo, that article, rather the author, is full of B.S., sorry. He makes several statements that are incorrect and misleading, I could not get through the entire article.

    Here are much better and correct explainations concerning horsepower and torque:

    http://auto.howstuffworks.com/horsepower1.htm


    http://www.engineersedge.com/power_t...ion_menu.shtml

  8. #8

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    Agreed. I meant to only reference the original Q regarding HP and Torque. Your link is better and has a T and HP graph that illustrates what I was trying to explain.

    Quote Originally Posted by .338WM View Post
    Cizmo, that article, rather the author, is full of B.S., sorry. He makes several statements that are incorrect and misleading, I could not get through the entire article.

    Here are much better and correct explainations concerning horsepower and torque:

    http://auto.howstuffworks.com/horsepower1.htm


    http://www.engineersedge.com/power_t...ion_menu.shtml

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    Back in 93 I had a 30 by 10 boat built by Marina Boat Works and it had twin 5.7 yamaha stern drives 245 hp each. Top speed was 55Kt and cruse speed was 34kts at 3000 rpm miles per gallon 2.45. In 1998 I change out the Yamaha's to 170 Hp Volvo diesel 32 series my top speed dropped to 30kts and cruise to 26kts miles per gallon 2.25. In 2002 I lost on of my volvo diesels and could not get a replacement so I install 5.0 Mercruiser gas engines with Bravo III. Engines where rated at 260 Hp. Cruise was 30Kt and top end 46Kts 1.5 miles per gallon. All I can say is that there is more to it than changing out engines to achieve better performance. You have to figure in vessel weight, type of hull, gear rato's of out drives, types of props, the pitch of the props and the list goes on. If you are only using your boat a couple of hundred hours a year diesels will not pay for themselves. You would be better off installing out boards on a bracket and gaining more boat space. With the outboards you can get a 3 year warrenty and then get it extended.

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    It usually doesn't pay to convert a boat unless you have to repower (like you blew up your motor, or it is now a rust pile).

    Bravo to all the previous posters on covering the issues. Good post topic.

    Your question seems to be gas diesel nmpg vs $/nmpg. The diesel will go further on the same ammount of fuel, but sometimes the diesel is more expensive.

    The less HP required to move your boat with a diesel is also true due to torque.

    A final thought is my friend has a Maxweld with twin Volvo D6's and he has blown 2 DPH drives at a cost of $11k each, both times a month or so past their warranty. A gas stern drive can be had for much less.

    Come to think of it, does your boat currently have a Bravo III or Volvo duoprop? If not that'd be my recommendation. I had a 27'x10 Uniflite that was a pig until I replaced the single prop outdrive with a duoprop outdrive. Big performance and economy increase for very little dollar outlay (I found a used one for $2,000).

    Sobie2

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    Default Boat help

    Currently I have a old durprop 290 (I think). The person I bought the boat from had converted the single volvo prop with the 290. He also had the old 260 hp volvo 350 motor with a carb,. The 290 duoprop has the old electric tilt motor which mean the prop is either up or down. I have re-powered (3 years ago) with a 6.0 liter vortec, Marine Power engine. With some work we were able to connect the "old" outdrive to the new motor. However I do not know what the gear ratio is in the outdrive. I have tried different props and have not seem to come up with the right one yet. I have considered upgrading my outdrive but that cost $$ and I am not sure which outdrive go with. Volvo makes outdrives for smaller engines and larger (big block) motors but I haven't decided where the 6.0l falls. The 6.0l is supposed to produce 375 hp at a higher rpm.
    My boat is an older bayliner 2860 with a 10 foot beam. I love the boat but I am trying to find the right combination to get the best performance out of it.
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    Me thinks there is a little bit of confusion here. The original post was really asking “is a gas horsepower the same as a diesel horsepower” and the answer is yes, they are the same measure of power. If each is geared and propped optimally (and if they weighed the same), a 300hp boat will have the same top speed whether powered by one or the other. That said, there are many other considerations. For example, if you assume that a gas engine will have a good service life running at 60% power and a diesel at 75% power, than yes you can get by with a smaller diesel engine at desired cruise.

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    Rod123,

    Your outdrive probably is the 1.95 ratio and with the new 6.0 liter you ought to have the 1.78 ratio. The 1.95 was the correct ratio for the 350 but no the 6.0 liter. I had this same problem when I converted my 27' Uniflite. You can get a volvo upper in the 1.78 ratio and get it swapped out for cheap (used from your local marine shop or you can do it yourself). I ran out of prop choices with my 454 (7.4l). Changing to the 1.78 ratio dropped my rpms/speed by 500 rpms across the board, and also got me propped correctly and the boat really came alive.

    Sobie2

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    Sobie,

    Thanks for the advise. I wondered if my outdrive was part of the problem. I will switch to the 1.78 ratio and hope that helps with the boat performance. I really like the boat and the 6.0 liter helped but I think switch the upper gear ratio will allow the boat to perform better. I did not realize I could switch the upper part of my outdrive out I thought I had to replace the whole drive.

    Thanks again,

    Rod123

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    Member captaindd's Avatar
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    On changing the gears ratio's on the DPH A drives you change the gears in the lower unit not the upper side. I lost two drives this year and am currently running a mix set. One drive is a 1.69 with a G-5 prop and the other drive is a 1.76 with a G-6 prop. They run about the same rpm's and the performance is the same as if I was running the 1.76 drives with the G-6 props. You check the parts by going to Marine Parts Express web site and look in the schematics section.

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