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Author Topic:   Single Engine vs Twin Engine
Al_A_Buy posted 04-01-2004 01:51 PM ET (US)   Profile for Al_A_Buy   Send Email to Al_A_Buy  
Team—as I ponder the variables surrounding the decision to repower one question continues to nag me: With regard to top speed and hole shot (acceleration) how does a single high HP motor stack up against two smaller yet of equal horsepower motors? For example, a single 200-HP vs twin 100-HP?

I understand the increased maintenance, equipment, and safety, etc., discussions, but I am looking for some real world experience or empirical data (about top speed and acceleration).

Thanks - Al.

JBCornwell posted 04-01-2004 02:44 PM ET (US)     Profile for JBCornwell  Send Email to JBCornwell     
I think the big single would out accellerate and outrun the smaller twins.

Why? Drag.

The only advantages I see of twins are security and cheaper trolling (on one).

Red sky at night. . .
JB

prj posted 04-01-2004 03:22 PM ET (US)     Profile for prj  Send Email to prj     
I'll counter JB and propose that twins would out accellerate the big single due to the greater bite of dual props and more diameter in the water.

Single faster at top end from all I've heard or read.

Ed Stone posted 04-01-2004 04:54 PM ET (US)     Profile for Ed Stone  Send Email to Ed Stone     
Twin 135 Opti's versus 225 four stroke.
Top Speed:twins 43mph/single 39.2mph
0 to 30 mph:twins 8 Sec./single 13.9 Sec.
Range/fuel economy:twins 391miles/single 351miles
(Trailer boat Mag. 10/02)
jimh posted 04-01-2004 08:48 PM ET (US)     Profile for jimh  Send Email to jimh     
The question is somewhat of a loaded one, since it is impossible to compare a single engine of more than 300-HP to twins. So in any case where the twins are larger than 150-HP each there is no doubt that performance favors the twins.

In smaller twin installations, say perhaps comparing a single 150-HP versus twin 75-HP, the case could be made that in pure performance the 150-HP might be better.

Again, this is a loaded question because the notion that the twins must be not more in combined horsepower than the single places a limit on the situation that tends to favor the single.

But no one goes to the expense and bother of twin engines because they think they will work worse than a big single. Doesn't that more or less give the answer?

Al_A_Buy posted 04-01-2004 11:35 PM ET (US)     Profile for Al_A_Buy  Send Email to Al_A_Buy     
Jim - This is a real concern. I am very happy with the performance of the 185 HP Johnson currently installed but know that a repower is in the not too distant future. As I am often well off-shore (30+ miles), the wife would feel better if I had twins (no pun intended). Since my hull is limited to 200 HP, I am trying to understand what performance difference I might experience if I mount twin 90's. Not wanting to limit the discussion to these specific dimensions, I did try to ask a somewhat leading question.

Al W.

LHG posted 04-02-2004 12:52 AM ET (US)     Profile for LHG    
Before you said you went 30 miles offshore, I would have said to stay with a single, and with that lightweight hull, you only need a 150 or 175. With the high weights of the new clean engines, I'm beginning to think that for an inshore Classic 22' and less, single is the way to go. But with your use, and considering the somewhat narrow transom configuration of the old 21 Outrage, I think I would recommend a pair of slim and trim Yamaha 90 2-strokes. Anything else in twin configuration would be a squeeze plus a weight problem with clean technologies.

My guess is that the next power for my 18 Outrage will be a "clean" 150 single, since the high weight of clean twins is an impossible situation with current offerings.

cape_rover posted 04-02-2004 06:27 AM ET (US)     Profile for cape_rover  Send Email to cape_rover     
When you're out 30 miles and one engine fails (twins do make the boat half as reliable), do you think you'll get the boat on plane with a single engine in the hp range you are talking about? Even if you reprop out 30 miles (a risk in itself) you still have to overcome the fact that the prop/lower unit on the failed engine will likely cause water resistance when you are trying to get the boat on plane. Even with the failed engine tilted all the way up, the stern is very low when you start to plane.

You need to consider this scenario because if you blow your now only good engine tring to get the boat to plane with an oversized prop, with an engine that is off center, and with an engine dragging in the water - your dead on the water. Likely you will idle back to the harbor for 30 miles rather than risk blowing your good engine. If you agree, then you may want to consider a kicker.

Clark Roberts posted 04-02-2004 07:27 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
Al, if it may help your decision, your 21 Outrage will plane with no problem (and probably run around 30mph) with a single 90 hp 2 stroke. I ran a 1995 Merc 3 cyl 90hp on a 21 Revenge for several years and it worked well for me. The 90 Merc is an 84cubic inch engine vs. 71 cubic inches for the 90 Yamaha but the yamaha is lighter (and narrower as Larry mentions). Happy Whalin'... Clark... Spruce Creek Navy
Clark Roberts posted 04-02-2004 07:31 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
PS> Whaler literature lists 65hp as Minimum for all the vintage 21 Revenges/Outrages and that was in the days when engines were rated at the crank (currently engines are power rated at the prop) so that would be equivalent of about a 55-60hp modern 2 stroker for min power to plane and operate.... Clark... SCN
bkovak posted 04-02-2004 09:56 AM ET (US)     Profile for bkovak  Send Email to bkovak     
Al_A_Buy, I can answer this question as I had a '75 21 Outrage with twin (89) Johnson 90's. The twin setup was ideal for offshore use. In the bay, I would occaisionally just run on one motor and could get up to 30mph. Running both, I was getting mid 40's at WOT. With light load and calm conditions, I'll bet you could hit close to 50 if propped and trimmed right. If you are looking for security for going 30 miles out without sacrificing performance, go with the twin 90's. That hull flies with those motors. Brian
Plotman posted 04-02-2004 10:39 AM ET (US)     Profile for Plotman  Send Email to Plotman     
Here is how I look at the single versus twin argument. On a given trip, assume you have a 1 in 50 chance of having some kind of mechanical failure on an engine (real number is probably lower). With a single, you have a 1 in 50 chance of needing to be towed in. With a twin setup, you reduce that to 1 in 2500.

This analysis assumes of course that the engines truely operate independantly of one another - e.g. if you run out of fuel or have contaminated gas, it doesn't matter how many engines you have.

Where I am, there is no such thing as Sea-Tow. I could easily be 50-plus miles away from the nearest coasties. And if I do call them, they are going to pick me up, but leave the boat. I have also been on the lake when summer storms have taken wind speeds from under 10mph to 80mph in a minute. (I know someone who had a 23' SeaCraft sink in that storm when he took waves over the transom).

In other words, for me the single versus twin argument isn't about performance.

Plotman posted 04-02-2004 11:58 AM ET (US)     Profile for Plotman  Send Email to Plotman     
Look at the performance data on whaler's website for the Outrage 24 - they compare a single 225 of both 2s and 4s varieties against twin 115 4s.

The Twin 115s come out of the hole faster than the single, and top out at a higher speed. I think this is especially interesting to note given that the performance data for the Nantucket and the dauntless 18 show that the 115 4s is a relatively weak engine (way below the 115 classic on the Nantucket).

Also interesting that the 225 opti and the 225 4s give almost identical performance on the Dauntless 22, but on the Outrage 24, the Opti has an edge.

DaveH posted 04-02-2004 01:20 PM ET (US)     Profile for DaveH  Send Email to DaveH     
Al-A-Buy:

Your question of resisitance cannot be answered to a difinitive number, however based upon some Naval Architecture references I have, you will experience 7-8% of your total hull resistance from the twin engine's underwater appendages. Similarly, you will experience about 3-4% of your total hull resistance from the single engine installation underwater running gear.

There are many more factors and plenty of ways to overanalize this question. However, in keeping the simplicity, it is my opinion that the extra drag from the twin engines is not that much and should not prevent you from installing them.

The question of pure acceleration is also difficult but the twin engines will be faster on plane in my opinion for these reasons:

Prop surface area: You will be able to apply more power to the larger prop surface area of the twin installation without cavitatiing the water

Torque: Smaller diameter props require less torque to turn and apply/use effective horsepower more efficiently. This phenomenon is related to the prop surface area as well.

I would definitely use twin engines for your typical use if the budget allows. LHG mentions the Yamaha 90Hp 2-stroke and I agree but would also check out the 90Hp ETEC if the on-center dimensions allow twin installation on your transom
-Dave

Outrage22Cuddy posted 04-02-2004 02:12 PM ET (US)     Profile for Outrage22Cuddy    
If & When I repower, it will be with a single engine with a kicker. I have twin 130's & the boat goes as fast as I could ever want. If you want a 22' boat to handle like a Sea-Doo, the twins will do it. I pass everything but a gas station. I would gladly go slower with a 150 or 200 single 4-stroke for better mileage. I live in So. Calif. where gas is well over $2 & is supposed to go near $3 by the end of the summer. As for running on a single in an emergency, you will likely be running back unplaned anyway wether you have 10hp or 150hp. I tried to plane with one motor raised & it did, but really straining. I would not want to run like that for a long distance.
LHG posted 04-02-2004 02:17 PM ET (US)     Profile for LHG    
From Brian's note about using twin OMC 90 degree V-4's on the hull, I guess transom mounting width would not be an issue with any current 90. I had always thought the transom was quite narrow on that hull, but maybe not. My guess is that twin 90's will run about same top end as a single 150.

If you're talking carbed 2-strokes, the Mercury and Yamaha have a big rigging advantage with integral oil injection tanks. For clean, it looks like the E-tecs take it.

greyg8r posted 04-02-2004 10:45 PM ET (US)     Profile for greyg8r    
Outrage, Cape,

I have twins on my Grady. Yes, if you have twins and one konks out, you are coming back sans plane and it will stress the good engine. BUT, I much prefer going 8 knots on one 200 engine (my cruising speed on one engine) versus the top speed of a 10 horse kicker, if it is even able to overcome the current and waves to get you back;-(.

I'd go with the twins if you go offshore. Different story if you stay inshore.

Richard


jimh posted 04-03-2004 08:15 AM ET (US)     Profile for jimh  Send Email to jimh     
Even if your set up with twin engines (of equal horsepower) does not allow you to get on plane with a single engine, you are still farther ahead with twins than you would be with a large engine. If you are powered with a large single engine, when it conks out you are not going anywhere.

But you ask, "What about a big engine and a 'kicker?'" That is really a twin engine installation, just with unequal horsepower. Powering a boat with one large engine and a small auxiliary will cost about the same as powering with twin engines of equal horsepower, assuming you have remote controls and proper steering linkages in both installation.

This decision is complicated at present by the weight of most low-Emission (low-E) outboards. Nowadays, unfortunately, installations of twin mid-range horsepower (75-115) engines are made less attractive due to the heavy weight of these motors. A pair of either Mercury 115 or Mercury 90 Four Stroke engines would weigh 772 pounds. They are rated as 3-CARB Star engines.

I think the low-E weight champion in larger motors might be the Mercury 175-Optimax. It weighs in at 431 pounds. Another contender is the Mercury 200XS Optimax Racing motor which weighs 434 pounds (but available only in 20-inch shaft). Both are 2-CARB Star rated.

Comparing a set up of twin 90-HP engines (180 total horsepower) against a single 175-HP engine results in a 341 pound advantage for the single. This number will grow larger, too, when the extra weigh of an additional propeller, fluids, and other rigging is added.

If the weight of an auxiliary engine is added to the single engine installation, the difference is not so great. Let us say we add a Mecury 9.9 Bigfoot engine on the transom. This adds 128 pounds to our single installation, raising its total to 559 pounds. This is still significantly lighter than a 772-pound pair of 90 or 115 four strokes ( which would be 213 pounds heavier).

These are, you might say, weighty problems!

andygere posted 04-03-2004 03:22 PM ET (US)     Profile for andygere  Send Email to andygere     
One of the better threads on this topic is located here: http://continuouswave.com/ubb/Forum4/HTML/001837.html

It's worth a read.

Lopaka posted 04-03-2004 03:41 PM ET (US)     Profile for Lopaka  Send Email to Lopaka     
In operate in Hawaii with my 22' Outrage with a single 225hp. It has never failed. Is there really a significant safety advantage with a second engine (any size) over simply keeping your single in top shape and running proper fuel etc??
cape_rover posted 04-04-2004 08:17 AM ET (US)     Profile for cape_rover  Send Email to cape_rover     
For all those considering duel engines just realize that you likely will not be coming in on plane if one engine fails.

I don't know how a 90hp pitched for a duel installation could get a 21' outrage on plane. Does not make sense to me unless you use both engines to get on plane and then shut one off. You wont have both engines to get the boat on plane if one fails...

Plotman posted 04-04-2004 12:09 PM ET (US)     Profile for Plotman  Send Email to Plotman     
Cape Rover - I don't know why you keep saying that a single 90 won't plane a 21. Do you have any first-hand experience here?

Sorry, but been there, done that. My uncle had a classic 21 outrage with twin 70s. It would plane on a single engine. That boat will plane no problem on a single 90. Let me say this again - so there is no confusion - the classic 21 will plane on a single 90hp engine, even when propped for dual installtion.

David

bkovak posted 04-04-2004 01:33 PM ET (US)     Profile for bkovak  Send Email to bkovak     
I agree. Not only does the 21 plane easily with a 90 hp motor, it will cruise at a good speed and very economically. I know a lot of members here like to max out their whalers hp but, for fishing or just cruising around casually, a smaller motor is fine as long as you are not running offshore. The classic 21 hull is very decieving. It has a ton of room compared to a 16/17 and will run nicely on the same 90 horsepower. I have taken both out in choppy conditions and the 21 handles much better. A lot of folks may not like the old "banana" styling but, it is a whaler worth considering if you fish big lakes or inshore and don't have a big boat budget. Brian
bkovak posted 04-04-2004 01:33 PM ET (US)     Profile for bkovak  Send Email to bkovak     
I agree. Not only does the 21 plane easily with a 90 hp motor, it will cruise at a good speed and very economically. I know a lot of members here like to max out their whalers hp but, for fishing or just cruising around casually, a smaller motor is fine as long as you are not running offshore. The classic 21 hull is very decieving. It has a ton of room compared to a 16/17 and will run nicely on the same 90 horsepower. I have taken both out in choppy conditions and the 21 handles much better. A lot of folks may not like the old "banana" styling but, it is a whaler worth considering if you fish big lakes or inshore and don't have a big boat budget. Brian
bkovak posted 04-04-2004 02:09 PM ET (US)     Profile for bkovak  Send Email to bkovak     
Sorry for the double post. Also to clarify, I'm not recommending you power a 21 with just a single 90 hp, just that it will run OK on one. Brian
Plotman posted 04-04-2004 04:27 PM ET (US)     Profile for Plotman  Send Email to Plotman     
Lopaka- engines, no matter how well maintained, do break down. Juts because it hasn't broken down doesn't mean it won't. If you do have a problem, your specific boating location and situation (crowded area with lots of help, vs. off of a lee shore of ugly reefs or steep cliffs) is going to determine whether that breakdown is an inconvienience or life-threatening. Remember, with an outboard, it is very difficult to do anything to make repairs while you are hanging off the boat in a seaway.
cape_rover posted 04-04-2004 09:35 PM ET (US)     Profile for cape_rover  Send Email to cape_rover     
Brain, The point I'm trying to get across is that when you are 30 miles out, I doubt you would run your boat on plane in 2-4 waves (typical) with a single 90 hp engine unless you reprop. I do not have experience with the 21' outrage. I do have experience with a larger 25' boat with twin 150 V6s and getting that boat on plane with a single engine would not allow it to turn anywhere near the recommended RPMs.
Al_A_Buy posted 04-04-2004 10:49 PM ET (US)     Profile for Al_A_Buy  Send Email to Al_A_Buy     
Wow - As usual, the advise and information provided has well exceeded my expectations.

Based upon the above, I guess I'm leaning toward the twins, all else being equal. I do like the secure feeling of having no single point failure items in the critical path when far a sea.

Thanks a ton for the interesting and reasoned discussion.

Al W.

Bigshot posted 04-05-2004 03:26 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Al....I have some advise that may sound harsh but you know me well and I don't intend it to be.

If you were running that rig 30+ offshore, you need to have your head examined. A new single would be fine for running offshore, especially compared to the engine you have which all things considered.....not very trustworthy. Before I dropped $10k into twins(or single) for that boat, I would look for a newer hull, or an equivelent hull that has been repowered. You will never be able to sell that hull with new power for MANY years unless you do a complete restoration. Your 185 is equiv to a new 150 which is where I would be looking. I think a 200 might do damage.

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