250's or 300's???

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Trousertrout
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Joined: Sat Jan 14, 2017 1:02 pm

250's or 300's???

#1

Post by Trousertrout » Thu Aug 15, 2019 8:28 pm

Last question for you guys a gals. Actually that's most likely a lie. I have looked but can't find a figure on gph or mpg on a similyboat to mine. 30 foot, 8'6" beam, 6000lbs dry. I'm wondering for best efficiency am I better off going with 250's or 300's? Am I crazy to expect 1.8mpg with 4 guys and equipment?

kmorin
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Re: 250's or 300's???

#2

Post by kmorin » Thu Aug 15, 2019 9:34 pm

t'trout,

there are some online calculators- paid sites typically- where various marine performance calculators are available. I suggest you consider https://boatdiesel.com/ where there are some minimal fees to get access to their online calculators that might help with your inquiry?

It is my understanding, and probably wrong due to my tendency to "assume" that this site (above) has paid to mount these calculators? So the fees they charge go to afford the online calculators?

Otherwise I wouldn't mention them on Welders' site where it might redirect traffic. I subscribe and use these "estimators" myself in my design work.

Cheers,
Kevin Morin
Kenai, AK
kmorin

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welder
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Re: 250's or 300's???

#3

Post by welder » Fri Aug 16, 2019 9:04 am

No worries Kevin.
I'll take the 300s, no one has ever complained about to much HP on a boat.
Lester,
PacificV2325, Honda BF225
2386

kmorin
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Re: 250's or 300's???

#4

Post by kmorin » Fri Aug 16, 2019 1:11 pm

T'Trout, Welder,
glad I'm not posting inappropriately- I really don't like to redirect on someone's site!!!

'trout- that all-up dry wt seems low for a 30'er? 6k lb seems like it should be more in the 8 or even 10k lb range? I'm sure you know the spec.s but.... I'm wondering if this figure is really accurate? If so- a couple of 300's (or even 250's) would make that hull move pretty fast?

One thing that not all owners consider- or at least I don't read many remarks about the fact? IF you have any given mass to move- displacement to propel- the larger the engines' capacity the less they "work" at moving the mass/displacement.

If you look at the horse power to weight ratio? the closer you get to a small wt being moved by a single horse power, not only will that craft have higher speed potential - most importantly from a seakeeping point of view; the ability to accelerate, due to low displacement per horse power, means that maneuvering in any sea is more agile and therefore safer, less tiring, and more reliable.

Not focusing on going fast, that's a given with a 600 hp boat that weighs 6000 lb! But discussing the ability to steer with ease and confidence in a following sea, to be able to quarter a head sea and to choose any heading in whatever seas you're in. Less horsepower means less agility- requires more attention or concentration at the helms in larger swells or seas.

In a 30' long keel boat- like a commercial fishing hull with the inboard engine forward- say a displacement of 9-12k lb (?) or 20-25k lb loaded with an engine of 150 to 200 hp- these non-planing hulls wallow about in a following sea, and rely on the helmsman's anticipation of movement and heading. They have enough horse power to wt ratio to move along - but not enough to quarter in the trough to a facing wave face- or to turn at the crest to face the next roller on the opposite quarter- therefore choice of course is sort limited when they're in larger seas. With the lower hp to wt ratio implied by your figures you'll have no trouble in any sea state keeping the heading you want.

Just another couple of comments on the original post.

Cheers,
Kevin Morin
Kenai, AK
kmorin

Trousertrout
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Re: 250's or 300's???

#5

Post by Trousertrout » Fri Aug 16, 2019 1:49 pm

Gotcha. The weight is surprising. That is not rigged weight. I should had specified. So probably 7600 with motors. So what you are saying is that the surge between troughs will be lessened by more hp? I hope that's what you are saying. The dealer believes the boat will be in the high 50's low 60's with the 300's

kmorin
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Re: 250's or 300's???

#6

Post by kmorin » Sat Aug 17, 2019 12:18 pm

t;Trout,
I'm not suggesting the hull's pitch, roll, or yaw will be different in a given sea- the shape and size and displacement will remain the influences there. The hull's movements will be more easily overcome by power when the higher power ratio is used. Like the difference between the extreme example of running home using the kicker/trolling motor/ "get home" 15 hp geared down engine compared to running back with the mains fully operational.

So if the boat is coming into a head sea- and therefore the bow is pitching up- and the wave face is quartering to the keel- or line of travel- the boat's motion tendency will be to slew off to the side opposite the sea/wave face and will require more helm correction toward the wave to keep the heading. B U T with more power - reserve of hp that can be added at anytime- the boat can be kept on course more easily because you get fairly instant response to the loss of speed due to climbing a wave face- by simply countering the bow up attitude and slewing action with more power -thrust.

At any given hull attitude- bow up, bow down, rolled to one or the other side- having the capacity to add power will make the same boat more agile- more rapidly responsive to sea state induced changes in movement. The example of a forward engine keel boat with a very low hp-to-wt. was to suggest that the higher horses means more freedom of the helmsman to guide any given boat in any given seaway.

I'd expect to see the speeds suggested. The question is always; how tall a sea/wave can you comfortably maintain what speed? Sure is light wt hull! probably beneficial for good gas mileage?

cheers,
Kevin Morin
Kenai, AK
kmorin

Trousertrout
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Re: 250's or 300's???

#7

Post by Trousertrout » Sat Aug 17, 2019 10:54 pm

Ok. Thanks so much for your time. So your saying I will not suffer gas mileage by going with more hp?

JonH
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Re: 250's or 300's???

#8

Post by JonH » Sun Aug 18, 2019 8:34 am

My 28' Crozier with a single Yamaha 350 V-8 gets 1.35 to 1.45 Nmpg at a 25 to 26 knot cruise speed.

A friends 28' Crozier with a single Yamaha 300 V-6 gets 1.1 to 1.2 Nmpg at similar speeds.

Note - while the hull shapes are not the same (hulls are designed by different NA's) the weights are
within a few hundred pounds of each other.

My boat fully rigged with no liquids weighs 7,980 - we put it on a truck scale before 1st launch.
With 2 people, gear and gas I'm typically between 9,500 to 10,000 depending on fuel load.

Good luck with your new boat, post pics once you get it.

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gandrfab
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Re: 250's or 300's???

#9

Post by gandrfab » Sun Aug 18, 2019 10:00 am

Trousertrout wrote:
Sat Aug 17, 2019 10:54 pm
Ok. Thanks so much for your time. So your saying I will not suffer gas mileage by going with more hp?
You don't have to run the 300's as hard as the 250's to get the same performance.
You can possible burn less with the extra HP, that is where throttle discipline comes into play.
When needed the extra power is a blessing that you can hold in reserve.

Trousertrout
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Re: 250's or 300's???

#10

Post by Trousertrout » Sun Aug 18, 2019 11:09 am

Thanks everyone. I built my dream weekend getaway house on the shores of lake Erie. Then my 14 year old daughter three a wrench in my plans and told my wife and I that she wants to go to this expensive private highschool. So the house is up for sale (haven't spent one night in it). Once it sells I am putting a deposit on it. Just have to figure out who is hanging motors and picking out some over the top electronics. I never in my wildest dreams thought I would be dropping this much on electronics but I never would have thought that they can be so cool now a days

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gandrfab
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Re: 250's or 300's???

#11

Post by gandrfab » Sun Aug 18, 2019 12:07 pm

We have a for sale section, I have 2.5 acres listed in central Fl. Post the house. :)
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