Questions Re: Sea Knight or Two-Plus

Designs for inboard or outboard power

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tpelle
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Questions Re: Sea Knight or Two-Plus

Post by tpelle »

Anybody built or owns plans for these two designs?

I am interest the minimum power requirement for these two boats. The Glen-L web site only indicates maximum horsepowers - 100 hp for the Sea Knight, and 135 for the Two-Plus. In interests of fuel economy, does anyone have an idea of how small an engine would be able to power these two designs in a practical sense? I'm thinking in terms of getting up on plane and running, maybe, in the neighborhood of 15 to 20 mph with a "reasonable" load - two people with enough provisions for a several days of cruising, plus necessary boat gear. Towing water skiers, for example, would not be a factor - just economical cruising.

It would go without saying that the boats would be built with a eye on keeping their weight down to improve performance.

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Trilody
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Re: Questions Re: Sea Knight or Two-Plus

Post by Trilody »

From a look at the hulls, a good 40 hp outboard would push the Sea Knight along at a good cruising speed. If you went with 50 hp that would be plenty for the speeds you're hoping for. The Two Plus is a lot heavier and I think you'd need something in the 60 hp class - it might struggle with a 50 and a 40 would be just maxed out and probably only get you to semi-displacement hull speed.

That's my assessment. Hope this helps.

Richard.

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galamb
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Re: Questions Re: Sea Knight or Two-Plus

Post by galamb »

On any planing hull, under about 21' in length, the practical minimum horsepower is about 65% of maximum and if you want the boat to perform as designed you need to rig with 80% (optimal horsepower).

"Under-horsing" a boat will burn more gas than using the optimal horsepower because the (smaller) motor will work harder trying to get the boat on plane and keep it there. A planing hull gets it's best "gas mileage" when it's on plane and the motor is at maximum timing advance with some "throttle" left - on most outboards that's somewhere around 2/3's throttle - often referred to as the "cruise speed".

In theory about 32 horses would keep a 17 foot, 1600 pound boat "on plane" at about 17 mph, but you would need the low end torque of at least 50 horses to get it to plane in the first place and that 50 would "guzzle" gas trying to get it there and burn substantially more to keep in on plane (once you back the throttle off) than say a 75 horse which would allow the hull to easily "climb" up on plane.

In the above case the 75 horse would ultimately be more "fuel efficient" than the 50 would - it would plane quicker and barely "break a sweat" keeping it on plane.

(I estimated a "built to plan" Sea Knight would weigh in at about 1600 pounds, rigged, with two passengers, minimal gear, motor, battery, safety equipment and gas based on a 700 lb bare hull weight)

If you are looking for something that can use a realively small motor you may want to consider a "displacement" hull vice a planing hull.

A design such as the "Tug Along" will reach it's displacement speed (about 7 mph on the 18' version) with a 15 horse motor.
Graham

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Trilody
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Re: Questions Re: Sea Knight or Two-Plus

Post by Trilody »

Ditto re Graham's comments - a planing hull is for getting there faster; whereas displacement and semi-displacement hulls are for fuel economy.

tpelle
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Re: Questions Re: Sea Knight or Two-Plus

Post by tpelle »

Thanks, guys.

I keep bouncing back and forth between a low-horsepower displacement boat (a Stambaugh Redwing 21) and a planing cruiser. Although the Redwing would go for a long TIME on a given amount of fuel, assuming about a 1/2 gallon per hour, the distance covered in that amount of time would be very low at, say 7 mph. Consequently the "gas mileage" in traditional automobile-centric miles per gallon would be low. Something like 14 mpg.

But if fuel consumption were, say, doubled to 1 gph, but speed increased by 3 times to 21 mph, then overall your economy would go up to 21 mpg.

On the other hand, a super-fast big motored gas hog would not only be expensive to build and feed, but it probably wouldn't be much fun to actually ride in for any length of time. I've been on a few of those boats, and while it's fun for a little while, sooner than later the pounding and noise wears you out, and you need a chin strap on your hat to keep it from getting blown off.

So, to cut to the chase, I'd be looking for an engine that could get a Sea Knight up on plane, burn about 1 gph, and push to about the 20 mph threshold or maybe a little lower.

I guess it comes down to what you want to do. If floating around on the water is what you want, not really going anywhere, then a slow displacement boat makes sense. If you want to cruise - that is, actually go somewhere - then a faster boat would make it more interesting. As long as it wasn't TOO fast.

I live along the Ohio River, and have in mind doing some cruising. There's lots of picturesque river towns that would be cool to visit, grab dinner in a local eatery, then spend the night on the hook.

There has to be a "sweet spot" where speed, fuel-efficiency, and gas consumption meet. That's what I'm trying to find.
Last edited by tpelle on Wed Feb 02, 2011 12:29 pm, edited 1 time in total.

upspirate

Re: Questions Re: Sea Knight or Two-Plus

Post by upspirate »

Bob Maskel built a Sea knight and powered it with a Mariner.

He would be the one to talk to about this as far as the Sea Knight goes.

http://glen-l.com/gathering/2007-photos ... ing-16.jpg

He'll come in here eventually,or you can PM him.

He will be at The Gathering in Sept in Tenn,you can se and ride in his boat then.

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galamb
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Re: Questions Re: Sea Knight or Two-Plus

Post by galamb »

I work on Merc outboards but occasionally do some rigging work for a couple of dealers (pre-deliver prep stuff). One of the biggest customer complaints, after the fact, is the lousy performance they experience with the "pre-packaged" deal that they bought.

Up my way there are two main companies (Tracker and Legend) that put together boat/motor/trailer packages at what seems to be a super value price. One of the hottest models is the 16'6"-8"ish fish/ski. Until very recently it came equipped with a 50 horse 2 stroke Merc, often a canvas package on a nice painted trailer for about 15K. Looked like a wicked deal when compared to Lund, Princecraft, Starcraft etc that had list prices of closer to 20K for pretty much the same thing.

The big difference is the engine. A 50 horse on a 800-900 pound, almost 17' aluminum hull that is rated for 90 horses will leave the performance disappointingly "lacking". Of course if you are willing to drop the 5.5K to upgrade to either the 75 4 stroke or the Optimax, the deal isn't so great anymore and the price is now similar to the better "namebrands" that usually have superior fit/finish.

The moral of the story is - do your homework and be honest about your needs and expectations. You will never regret hangin' more horses off the transom than you think you will need, you can always go light on the throttle - if you go with the minimum horsepower, most owners will always be disappointed and wish they had spent the few extra bucks in the first place - because there is nothing you can do after the fact, save swapping motors, that will improve the performance to any noticable level.

Now, without contradicting myself, you still would have a somewhat viable 40 or 50 horse option. Merc builds both a 40 and 50 horse, 4 stroke "Bigfoot" model and Yamaha has a 4 stroke, 50 horse "High Thrust" model. These are designed to push very heavy hulls (primarily pontoon boats), at lower speeds without bogging out the engine. They have very low gears (you can think of them like the tow package on a truck), so the motor doesn't work as hard to produce the same torque as similar rated motor with higher gears.

So they lack in top end speed but have tons of "guts". They are more expensive than their (normal) counterparts - close to the price of a regular 75 horse model - but definately might fit your needs.

If however, you plan on running on bigger water, you still may want to opt for more horses incase you have to get off a nasty lake in a hurry. (which goes back to my comment about your needs and expectations)
Graham

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A "professional" is someone who gets paid for their work - it doesn't necessarily mean they are good at it :)

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Trilody
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Re: Questions Re: Sea Knight or Two-Plus

Post by Trilody »

I haven't been on the Ohio river, but on the several inland waterways that I've boated on here in the Great Lakes regions, there are a lot of speed limits. If that's the case where you plan to boat, then a fast boat is maybe not so useful???

By the way, I'm a Merc guy and like their engines a lot, both for performance and from what I've seen, pretty good resale value.

Richard

tpelle
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Re: Questions Re: Sea Knight or Two-Plus

Post by tpelle »

Trilody wrote:I haven't been on the Ohio river, but on the several inland waterways that I've boated on here in the Great Lakes regions, there are a lot of speed limits. If that's the case where you plan to boat, then a fast boat is maybe not so useful???

By the way, I'm a Merc guy and like their engines a lot, both for performance and from what I've seen, pretty good resale value.

Richard
No speed limits to speak of, except as it passes by some cities they have no-wake zones. In the "pool" (space between dams) where I live there is 99 miles of waterway and maybe one no-wake zone of a couple of miles as it passes by Cincinnati.

I live a couple of miles from the river, actually, but on summer evenings I can hear the go-fast boats from my house.

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Trilody
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Re: Questions Re: Sea Knight or Two-Plus

Post by Trilody »

Cool. Maybe you need a fast boat then?

I'd like to boat down through there someday. Perhaps when I do the great loop. For next Summer we're going down the Erie Canal and then the Hudson and back.

Hope to see your build here after you've selected one.

Good luck.

Richard

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Mr Hot Rod
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Re: Questions Re: Sea Knight or Two-Plus

Post by Mr Hot Rod »

tpelle wrote:But if fuel consumption were, say, doubled to 1 gph, but speed increased by 3 times to 21 mph, then overall your economy would go up to 21 mpg.
Those numbers may be overly optimistic. To get a better handle on fuel consumption, check out the GPH-MPH-MPG Charts on Mercury Marine's 40-60 HP Engine Tests (Click on the Engine Test tab).

Maybe Graham can comment on the test results.

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Bill Edmundson
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Re: Questions Re: Sea Knight or Two-Plus

Post by Bill Edmundson »

tpelle

I looked into this on another boat. I compared a 115hp to a 150hp. Using wt/hp ratio and manufactures info., I found that at a given speed there was very little difference in fuel consumption.

The 115 at wot would give 35 mph. The 150 at wot would give 40 mph. But, at 25 mph, the 150 at lower rpm use about the same as the 115 at a higher rpm.

The big difference is up front cost.

Bill
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tpelle
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Re: Questions Re: Sea Knight or Two-Plus

Post by tpelle »

Thanks for all the replies. The difficulty with trying to estimate gph from all of these charts is that most of boats using engines around 50hp to 75hp seem to be pontoon boats! I guess nobody makes production monohull boats any longer in that size and weight range.

I've been looking at "legacy" designs, such as Sam Rabl's Puffin, an 18' cruiser quite similar to the Sea Knight, and he recommended something like, if I recall correctly, 15hp to 30hp. I know that modern fiberglass boats are much heavier than a plywood on frame boat, so I was thinking that maybe a Sea Knight could be similarly powered. I guess those 1950's horses must have been bigger, huh?

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Re: Questions Re: Sea Knight or Two-Plus

Post by Dwain the ski king »

In 1960 my shop teacher built a Sea Knight in high school shop, and of course the biggest motor then was 40hp Johnson, and he SKIIIIIIIED with it... He was 200# and it would pull him just fine...Top speed was about 35mph...SOOOOOO, 50-70hp would be a good motor for it...Bob,s(Maskel) Sea Knight ( I think) goes about 45 with 100 horse Mariner, and it gets on a plane pretty good.....Come on Bob, damn it, you aren't gonna let a little snow keep you from chimin' in here...PTTTTTTTTTTTTTTTTTTT, Dwain, The SKIIIIIIIIIIIIIIIII KIIIIIIIIIIIIIIIIIIINNNNNNGGGGG.....

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darthplywood
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Re: Questions Re: Sea Knight or Two-Plus

Post by darthplywood »

if you look at some of the old Glen-L catalogs they are powering a Sea Knight with a 25 horsepower Evinrude, and it is up on plain!

I myself am going to power mine with a 90 HP evinrude etec...but i want some speed...
Built the 17' Glen-L "Sea Knight"
yet to come...11' Glen-L "Utility"

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