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Weight and power

vtmechengvtmecheng Posts: 622 Crazy Baller
How much weight would need to be removed from a modern ski boat to make 300 hp acceptable instead of the 400 hp in some current models? Would 500 pounds be enough? Also, what would this weight reduction do to the wake?

In looking at boats and weight I realized that the engine is about 900 pounds of the weight dry. Now compare that to the Evinrude eTech weight at 500 pounds, which includes the transmission, lower assembly, and prop. Before you flame me for talking about outboards, please hear me out. I'm thinking the eTech engine could be mounted as an inboard. It puts out a lot of power at a low weight and size, reducing boat weight and increasing usable volume. I'm sure there are some serious drawbacks. What an I missing?


  • DWDW Posts: 2,059 Mega Baller
    I have done a significant weight reduction program on my boat with excellent results, 500 makes a nice difference. To your comment, the location has a significant impact on wake improvement, specifically transom weight. Overall also helps as long as you do it all at or rearward of the CG. Also, makes the boat much more nimble.
  • HortonHorton Posts: 28,321 Administrator
    @vtmecheng I asked the same question a number of times years ago. The answer I got from smart guys is those little light power units that push an outboard are fragile and complicated and will not run for nearly as long as a big heavy iron block V8.

    I believe the technical phrase is "there is no replacement for displacement".

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  • skibrainskibrain Posts: 107 Baller
    225 - 300 hp Etec is 3.4L
    Yamaha XTO (425 hp) is 5.6L but 952lbs.
    Big Merc is turbo or SC.

    Etec, especially laying on its side to spin an onboard drive shaft would be a physically smaller pkg. Fewer moving parts than v8 inboard. Could be interesting. Hole shot great. Less hull displacement with less weight. I know we run with 1/4 tank gas and back seat out for less weight in a DD. Not 500lb, but noticeable.
  • Tom351Tom351 Posts: 110 Baller
    edited March 2019
    I think the inboard V8 is there more for the fairly flat torque curve than just the peak HP number. I don't have the torque curves in front of me (HP is just mathematical torque) but even though a 300hp outboard may have 75% of the HP (compared to a 400hp inboard V8) at 5000RPM- it might only have 40-50% of the torque at say 2000-3000rpm. That would really slow down the "hole shot" unless you are talking about reducing the weight of the boat by 50%. You could add a transmission with gears to get the RPM up more quickly but that is just weight and complexity.
  • ALPJrALPJr Posts: 1,987 Mega Baller
    edited March 2019
    A Malibu Flightcraft with a 200hp Mariner has been in our rotation for 20 seasons with over 1,500 hours and still running strong. Spark plugs, gear oil and a battery only maintenance so far
  • vtmechengvtmecheng Posts: 622 Crazy Baller
    edited March 2019
    Thank you all for the great responses so far. There's no way a 4 cycle outboard engine would work, only a 2 cycle would have a chance in my mind.

    Torque: I agree that something like an eTech isn't going to have the same flat torque curve as a big V8. 2 cycles are better at this then the 4 cycles though. I'm not sure what is truly needed here because all ski boats have more than we need. If the driver punches the throttle, the handle would be ripped from the skiers hand. Maybe the eTech isn't enough but I have read a few on here with them say the holeshot is strong, that could be exaggerated. Maybe the weight savings would be enough to offset this difference.

    Weight: Would taking weight out of the engine location actually be bad for tracking or could that be overcome by hull design? This seems like the a big question.

    Longevity: I think this is the possible lowpoint of the small engine. There's the argument of fewer moving parts in the 2 cycle and I've seen some real old outboards keep running. It would take some digging but I'm sure there are engine hour discussions on fishing forums. If it lasted 1500 hours or more I would say that's reasonable.

    Honesty: I can't help but think that part of the reason for the big V8 used today is simply perception. It's cool having a huge V8. They sound cool, look cool, and are cool. I'm a gear head and totally get it. Maybe that would be muted quickly if the smaller engine really did improve wake and/or efficiency while still having the power needed. Ford still has some that don't like the Ecoboost and want a V8, to the extent that they offer the 5.0 even though it would likely be more profitable if everyone were ok with a turbo 6 and they could drop the extra option.
  • vtmechengvtmecheng Posts: 622 Crazy Baller
    @Tom351 I was having a hard time finding torque curves for compassion. Completely agree that torque at lower RPM is a big player.
  • BraceMakerBraceMaker Posts: 3,925 Mega Baller

    These new ski boats don't need the power for the weight. But these new hulls are plowing and spraying water out and forwards which you can clearly see if you drive one next to a 90s boat.
  • Tom351Tom351 Posts: 110 Baller
    @vtmecheng I just found some curves:

  • vtmechengvtmecheng Posts: 622 Crazy Baller
    edited March 2019
    @BraceMaker wouldn't less weight mean the hull sits higher so drag is lower? Buoyancy is just displacement. Modern boats are a bit wider but not much longer. Therefore, any additional weight over old boats has to push the boat down until it plans. If the boat is higher in the water it can plane faster. I worked on a Marine Corps amphibious landing vehicle and the weight coupled with limited planing surface killed hole shot (I did nothing with hydrodynamics). We were doing everything possible to just save 50 pounds. I would think a 500 pound savings would make a big difference in a ski boat's time to plane.

    The hole shot is where the low end torque is needed. If the boat sits higher less torque should be needed for the same hole shot. Once on plane and the engine is spun up, low end torque isn't needed. There's a lot going on and the answer isn't simple. It's too bad there aren't reps from the big three here to answer questions like this.
  • vtmechengvtmecheng Posts: 622 Crazy Baller
    @Tom351 nice find. So it comes down to if the 100 lb-ft of torque is offset by the weight reduction.
  • ReallyGottaSkiReallyGottaSki Posts: 84 Baller
    Cool topic, and smart dialog men

    Funny, I've long often pondered the same regarding a 2-stroke v6 powerhead with gear reduction in a direct-drive boat.

    I suspect maybe modern boats have encountered some bloating that is making base 300hp feel flat? Don't know, i'm not in many!

    Some may concur a tired 240hp v8 can pull slalom fairly well when the boat is not bloated

    All towboats surely using only a fraction of available power that is available during a pull, as evidenced of available throttle and manifold vacuum. Perhaps, only 90-110hp used to maintain cruise at 36, depending upon the hull?
    And such an engine at 3600 rpm, (given about 1mph/100rpm typical) only has maybe 160-170 hp available at 3600 rpm no matter what.
    So the larger that delta, between what is utilized at cruise, and what is available at that rpm ,(which is not max rated power, but dictated by the power curve) Of course slippage and compensation on firm tugs, and increased throttle inputs raise rpm to where more power (and therefor torque) is 'available' in reserve because rpm has increased. But its never going to be full rated power, because we don't ski at those speeds.

    So that said, max rated hp is not all available at ski speeds

    Interesting for the discussion, i've driven slalom with a 'not advertized' 425HP 2001 nautique (with the LH rotation High rpm barefoot engine, capable of 5400 rpm, and one can feel a base 351 is more responsive to throttle compensation at slalom speeds.
    Now that configuration of nautique has a 14p prop so was at not the same rpm, and must be disclosed.
    One can surmise a higher max rated hp at max rpm does not automatically guarantee the superior experience at lesser rpms.

    As been said prior, Available delta-torque/per throttle input at the ski rpm could be what we define as a 'responsive' tow tractor.

    I concur, a 225-250hp 2 stroke powerhead with smartly selected gear reduction would make an interesting project for the tinkerer.

    Recently, before undertaking my engine project, I've done some mental math on a economical lightweight LS based powerplant transplant, carburated, for older hulls, based on a 5.3 all-aluminum LM4 from a trailblazer ext or its GMC variant, done with a flywheel spacer, a readily available LS intake and spark box, and aluminum cast exhaust manifolds are becoming available.
    I've seen entire running vehicles on craigslist for 5-600 bucks. Given all that, i think the project would cost about 3300 max and get a couple hundred pounds out of an 351 or 350 or LT1 based boat.
    I've considered it for my boat, and i bet taking a couple hundred pounds out of say, an already stellar 20 year old Malibu response, is quite tantalizing.

    I'm currently in the project of 'aluminizing' my tired 351, and paired with ss commander manifolds i've recently procured, will take 141 pounds out of the boat total. Longer range plans are to prepare a 331 stroked shortblock for additional 60-70 pounds lost
    The boat already has a fantastic ski wake even by modern standards and excited to see how it is this season.

    to sum up, i may have a data point to share regarding such in a few weeks
    Wake and side spray observations will be observations but the seat of the pants data is tainted because power will be 'enhanced' as well :D

  • Orlando76Orlando76 Posts: 1,111 ★★★Triple Panda Award Recipient ★★★
    Hate outboards. Hate two strokes. Really hate the two together for a ski boat. My 351’s might have more moving parts than an Etec but since I’m still running Windsor blocks, the repairs and maintenance must be minimal.

    BUT.... I think the question to ask is what kind of ski boat would become if the outboard was mounted in middle of the boat, vertically like normal only in the middle? Would tracking be worse or better? Hole shot?
  • ReallyGottaSkiReallyGottaSki Posts: 84 Baller
    expand on that, why considering vertically vs inline?
  • DWDW Posts: 2,059 Mega Baller
    As far as tracking, I have not noticed a loss in tracking with a significant amount of weight taken out of my particular boat (375 lbs.). One caveat, the CG on my project moved significantly forward which actually improved the tracking. I thought about the potential, and was thinking what would be needed to compensate if that happened, I thought maybe larger tracking fins if needed.
  • UWSkierUWSkier Posts: 1,245 Mega Baller
    Big two strokes can last a hell of a long time. My dad had a 1988 Merc 175 on a lightweight Concord. With a 21p prop, it'd hit 70 MPH new. Guy who bought it is a friend. It's only ever had plugs and batteries. It'll still hit 70 MPH and runs great.

    A DI 2-stroke powerhead mounted horizontally would be cool. Would need to figure out a better transmission setup than the outboards use though.
  • ReallyGottaSkiReallyGottaSki Posts: 84 Baller
    I concur

    Back then the merc 150/175 powerheads would just run and run forever
    But seemed there was always a 200/225's in the shop all apart for rebuilds.

    Think would have to use a typical gear reduction B/w, pcm or zf to absorb the shaft's thrust and transfer it to the stingers/hull.
  • eleeskieleeski Posts: 3,937 Infinite Pandas
    The weight saved with a 2 stroke will be offset by the extra fuel you need to carry for the increased fuel burn. Plus, it will be difficult to meet emission standards. 2 stroke is not a realistic modern option.

    Hybrid electric might work. Maybe not save weight but certainly allow flexibility in where you put the weight. Possibly user adjustable - forward for slalom and back for tricks.

    Lighter boats would be awesome for towing.

  • dvskierdvskier Posts: 543 Crazy Baller
    I agree with Eric, two stokes are environmentally undesirable. I understand that improvements have been made in two stroke technology but you still are casting I burned fuel and oil into the lake. In the future I believe they will be not allowed, example Lake Tahoe already in effect.
  • BraceMakerBraceMaker Posts: 3,925 Mega Baller
    Its an interesting question - 500 pounds will certainly change how deep a boat sits when not in motion - easy to measure put a few extra people in and look at the freeboard. 500 pounds of weight needs to displace 500 pounds of water - about 60 gallons of 8 and some cubic feet of water. If you could measure the change on freeboard in inches you could then back compute the effective surface area of the running surface of the boat.
    But that's just a static displacement issue - having less will make a hull easier to get into motion.

    Once moving you have a different scenario - to me that question would be of the total drag through the water how much impact does 500 pounds have? Definitely the CG of the hull is critical both for safety and handling, but I think the modern hulls have more wetted surface, more surface features that promote drag, and are therefore less weight sensitive than the older versions. For instance ski 15' behind a 1980's stars and stripes with 2 people, then repeat with 4 people sitting in it. ~300 extra pounds generates a huge delta in wakes. Same thing with a modern boat? You barely notice tossing an extra couple adults into a new prostar.
  • Orlando76Orlando76 Posts: 1,111 ★★★Triple Panda Award Recipient ★★★
    @ReallyGottaSki like a “mullet Skiff” there’s some advantages to this design over the mainstream transom mount outboard. Better stability, better efficiency, lighter transom things similar to the pro’s of a DD inboard. Would a smaller outboard in a mullet skiff put the torque curve in a more desirable spot for skiing? Put the outboard forward and tracking fins on the transom for tracking. A design like that is really thinking outside the box.
  • vtmechengvtmecheng Posts: 622 Crazy Baller
    I love the thought coming out of this. The emissions is an issue. The current eTech is approved for most lakes because it is better than other 2 strokes but that doesn't make it better than a 4 stroke.

    On weight and it's impact on how the boat is once on plane, I just don't know. Everything I've ever calculated had to do with how much power is needed to get the vehicle to plane. Less weight has to help though.

    I had thought about the transmission and that it may get heavier because more reduction is needed. On the other hand, less low end torque means it doesn't have to be so strong.

    @Orlando76, I like your out of the box thinking.
  • ReallyGottaSkiReallyGottaSki Posts: 84 Baller
    @Orlando76 , heh, i looked up mullet skiff and expected to a see an aquatic IROC-Z...
  • Orlando76Orlando76 Posts: 1,111 ★★★Triple Panda Award Recipient ★★★
    @ReallyGottaSki ha, would make sense since they’re most common in rural FL, Miss and LA.
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