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Author Topic:   max engine size?
jacko posted 09-20-2002 04:47 PM ET (US)   Profile for jacko   Send Email to jacko  
Ive got a 2000 OR 18 and I'm interested in changing the engine from a 175hp to a 225hp. Unfortunately the BW spec. says it is limited to a 200hp so I'll probably invalidate the warranty if I did this. As far as I can see the Yamaha 175 and 225 are the same engine block and weight so whats the problem? Will this put extra load on the transom even though its the same weight?
lhg posted 09-20-2002 05:09 PM ET (US)     Profile for lhg    
If you want to stay within the warranty, consider a Mercury 200 Optimax. It's probably the fastest 200 out there, since it's basically a 225 Optimax in disguise.
They are saying the new 200 Fichts are pretty fast also.

If you just want brute 2-stroke HP, get a 225 Merc EFI and be done with it. It's really closer to 250HP.

I'm assuming you're already factory rigged for Merc. If not, stay with Yamaha for the same reason.

Peter posted 09-21-2002 07:03 AM ET (US)     Profile for Peter  Send Email to Peter     
The Yamaha EFI 175 is built on a 2.6 liter block and weighs approximately 450 lbs. The Yamaha EFI 225 is built on a 3.1 liter block and weighs approximately 500 lbs. The 225 should put more load on the transom. Can the 2000 OR 18 take it? Don't know.
Swellmonster posted 09-21-2002 02:23 PM ET (US)     Profile for Swellmonster  Send Email to Swellmonster     
You may also have problems insuring it too.
NoviceWhaler posted 09-23-2002 01:01 PM ET (US)     Profile for NoviceWhaler  Send Email to NoviceWhaler     
There also might be local statutes that limit your HP.
roofer posted 09-23-2002 01:23 PM ET (US)     Profile for roofer  Send Email to roofer     
Jacko, why do you want to do this? I have a 2000 18ft outrage with a 135 opti. Just curious?
David Ratusnik posted 09-23-2002 02:19 PM ET (US)     Profile for David Ratusnik  Send Email to David Ratusnik     
jacko- Sounds like you enjoy speed. Sounds good to me. Rec- put a light Armstrong or custom single bracket on the boat, then dangle the bigger engine on the bracket plus add hydraulic. The bracket should help the top end speed with the added horsepower. See brackets section on this Forum. .03 David
David Ratusnik posted 09-23-2002 04:26 PM ET (US)     Profile for David Ratusnik  Send Email to David Ratusnik     
Check with a custom bracket designer. .03 David
jimh posted 09-23-2002 11:53 PM ET (US)     Profile for jimh  Send Email to jimh     
The maximum rated horsepower on the rating plate is required by the rules to be derived in a certain proscribed manner. In other words, the rules say that if a manufacturer has to put a rating plate on the boat, the plate has to have a recommended horsepower maximum, and that figure has to be computed using the techniques described in the rules.

The manufacturer can't invent his own technique to measuring the maximum; he has to use the one in the rules. This is reasonable as all boats with horsepower ratings will then at least be comparable in the sense that those ratings were developed in a similar computation.

It may very well be that a hull could tolerate more horsepower than shown on the rating plate. As I recently posted in another article, the USCG does not require a boat to be powered at no more than the rating plate figure; you can use a larger engine. (I hesitate to say "over power" because it is not clear that the boat will be over powered, it will just be powered over the rating.)

The warranty coverate should be spelled out in the warranty. I would hope that it would explicitly state the coverage (or lack of coverage) of the warranty with regard to having an engine above the rating.

I don't know what the warranty says, but it would be interesting to have it included in this discussion. It is always better to get first-hand information right from the source.

jacko posted 09-24-2002 06:35 AM ET (US)     Profile for jacko  Send Email to jacko     
Roofer
Why? Why not!
I agree that the 2000 18 OR is a fantastic boat and even with the 175 it runs with at the moment is fast but dont you ever wonder what it would be like running at its max capacity? 50-60 knots maybe!! I have taken mine out on some really long runs in very big seas and I know the boat is along way from what it's capable of. It's like buying a racehorse and trotting it around the paddock!!
I dont know anything about the warranty of the hull. How long is it for? what does it cover? - I'll find out from BW.
roofer posted 09-24-2002 10:01 AM ET (US)     Profile for roofer  Send Email to roofer     
Sounds great, when you get it done please let me know how the ride turns out. 50-60 knots WOW!!!!!
Bigshot posted 09-24-2002 10:38 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Jacko....you're wacko! It rymed so what the hell.

seriously, 50hp aint gonna do crap. You might pick up 5mph and a better holeshot. I think you are having dillusions of grandure.

dhlaw posted 09-24-2002 04:36 PM ET (US)     Profile for dhlaw  Send Email to dhlaw     
I remember reading that hull design is the ultimate factor in speed limitation (hydrodynamics) and that no matter how much power you put on a boat it will only go so fast. That outrage isn't going to run 60 knots....unless you drop it from a plane!!
Bigshot posted 09-24-2002 06:07 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Maybe a 300 Promax but I would not want to pilot it. Maybe 60 at that....not knots either.
jacko posted 09-25-2002 03:20 AM ET (US)     Profile for jacko  Send Email to jacko     
I dont know a thing about hydrodynamics but it seems to me that if I put 500hp on the back of an 18 OR I could get it to go 60 Knots. Assuming the thing doesn't sink or fly!!
I can do 44 Knots at the moment and you are probably right in saying that I'm not going to do 60 with a 225hp engine - nice dream though.
If I ever fall to temptation and stupidly order a larger engine you will be the first to know the results.
Do other people crave things they know they dont need or want?
Jacko
Bigshot posted 09-25-2002 10:00 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Never! Supermodels have never entered my mind:)

A hull has a maximum speed that it will hit. In order to break that speed by 10mph you usually have to double the HP. This is great with I/O's where the weight does not change. Putting 500hp on a 18' boat would be impossible. The 900lbs of engines would dramaticall effect performance and loss of control would probably happen before top speed would.

Richard Quinlivan posted 09-25-2002 10:39 AM ET (US)     Profile for Richard Quinlivan  Send Email to Richard Quinlivan     
Increasing your Horsepower from 175 to 225 will raise your top speed by the square root of 225 divided by 175 all other things being equal. That is about 13%. Obviously lots more weight will negate some of the HP increase.
Planing hulls don't have a max hull speed only displacement hulls. The limitation would be controllability. Porpoising and/or chine walking.

To increase top speed more move your cg aft. That will let you trim higher for more efficient operation at the higher speed. It will also hurt your hole shot.

Dick

Bigshot posted 09-25-2002 11:27 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
They do not have a max speed per say but like I said....after a certain speed you will have to double your HP to gain 10mph. Unless I misunderstood Reggie and Steve Stepp.
roofer posted 09-25-2002 12:37 PM ET (US)     Profile for roofer  Send Email to roofer     
Richard, that was an interesting and potentially very usful fact. i am haveing a hard time following the math. please help
225 sqroot = 15 the 15/175=9% is this correct. would this mean if the previous top speed was 40 mph the new top speed would be 43.6 mph?
Richard Quinlivan posted 09-25-2002 02:13 PM ET (US)     Profile for Richard Quinlivan  Send Email to Richard Quinlivan     
Sorry my equation got garbled by my words. Divide 225 by 175 equals 1.286. Then take the square root equals 1.34. Therefore 40 mph would go to 45.4 mph.

New speed = Old speed times Square root(New HP/Old HP. Double HP and get a 40% increase in speed. Obviously you need to prop the new motor correctly, not add to much weight, trim it right, not make the boat so fast it can't be controlled, etc.

Dick

jacko posted 09-25-2002 03:28 PM ET (US)     Profile for jacko  Send Email to jacko     
Dick
Thanks for the info - very useful formula.
I think i'm right in saying to achieve 60knots I would have to put a pair of 200's on the back of my 18 OR !!! I dont think , even if the engines could fit, BW would honour the warranty agreement!
Bigshot posted 09-25-2002 03:38 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Hahahaha!
roofer posted 09-25-2002 05:50 PM ET (US)     Profile for roofer  Send Email to roofer     
Dick, thanks for clearing that up.

do you think this fomula gets more out of wack with a big jump in, i.e lets say I have a 50 and can go 25 mph and i jump up to a 200 the formula 200/50= 4 take the sp =2 times the oringinal speed 25mph=50 mph should the formula run as true for a big jump as say a jump from 150 to a 175. assuming all the prequalifiers of prop, and set up ect?

lhg posted 09-25-2002 05:57 PM ET (US)     Profile for lhg    
Those interested in this subject should see JimH's Perfomance Predictions in the Reference Section.
DaveH posted 09-25-2002 06:14 PM ET (US)     Profile for DaveH  Send Email to DaveH     
Dick:

I know we are talking generalities here, and maybe I am missing something, but where do you get this equation from? I am having a difficult time with the over simplication of may factors that determine planing hull speed.

Richard Quinlivan posted 09-26-2002 09:29 AM ET (US)     Profile for Richard Quinlivan  Send Email to Richard Quinlivan     
This formula is available from a number of places. For example in the Reference section of this Forum under Propulsion, Part 2 is a section on Calculating Speed Potential using a formula by Clark Roberts. If you use that formula to calculate the Speed potential of a hull with two different HP motors you get the equation mI provided. It is also the formula used in a Mercury performance slide rule I used to have. I think that a formula like Clark Roberts' is in a book by Dave Geer on propellers. It is home and I am at work so I'll check later on that.

The hull factors are more or less accounted for by starting with a given boat speed for a given HP. All you are calculating is the potential speed change for a given change in HP. A large HP change would imply a large weight change. A large potential speed chande implys a need to get to optimum trim for max speed. All these formulas are approximations because of a lot of unknowns. If predictions were perfect we wouldn't have boat races, or car races, etc.

Dick

DaveH posted 09-26-2002 07:55 PM ET (US)     Profile for DaveH  Send Email to DaveH     
Dick:

Thanks. I understand now what you're doing. Brain failure on my part. Although you probably already know, the relationship is only good for comparison of 2-strokes or 4-strokes, but not 2-stroke vs. 4-strokes since their torque curves are quite different. Good information though.

Richard Quinlivan posted 09-26-2002 11:56 PM ET (US)     Profile for Richard Quinlivan  Send Email to Richard Quinlivan     
Dave

Since all the equation deals with is top speed potential it doesn't care how you make the horsepower. The process of getting to that speed has a lot to do with the torque curve as the engine and boat accelerate but the end speed won't care.

Dick

Richard Quinlivan posted 09-27-2002 12:08 AM ET (US)     Profile for Richard Quinlivan  Send Email to Richard Quinlivan     
I looked up the reference I mentioned to the boat speed formula as provided by Clark in the Reference Section. It is in Propeller Handbook by Dave Geer. he calls it Crouch's Planing Speed formula. The fudge factor is the constant for different hulls given in Clark's formula. BTW this book will tell you more about boat propellers then you ever wanted to know.

Dick

Vek posted 10-04-2002 04:30 PM ET (US)     Profile for Vek  Send Email to Vek     
In general, regarding anything that "drags", be it a boat in water, plane in air, automobile through air, water through pipe, etc., to increase your speed by a given factor (say, double or 2x) you need that factor squared more power (quadruple or 4x). This is rough but usable - if the speed change of the device in question means going from one operating regime (displacement for boats) to another (planing for boats), the rule won't hold; there's a power spike then drop in getting on plane. But, for where you're talking, it should be close. Better suited for fast-moving planes, automobiles, or trucks.
Richard Quinlivan posted 10-04-2002 11:04 PM ET (US)     Profile for Richard Quinlivan  Send Email to Richard Quinlivan     
The complicating factor in determining the drag force change on a planing boat as speed increases is that the area of the bottom in contact with the water is reduced in proportion to the square of the speed of the boat.
Since the drag force is proportioal to the product of the area times the square of the speed the drag force due to speed ends up more or less constant as the speed changes.

As you go faster the drag of the lower unit starts to be the major factor in drag force. That is why raising the lower unit out of the water imprioves speed.

Dick

homey posted 10-08-2002 01:30 AM ET (US)     Profile for homey  Send Email to homey     
Jacko, My friend has a 1986 outrage with a 200hp carb Mercury(1995). It tops out at 52mph and cruises nicely around 40mph.(gps)He modified the transom slightly by placing splash guards on both sides of the engine/transom. This prevents alot of water coming into the boat/splash well when drift fishing etc.
He actually wishes he had the 250hp and believes the boat can handle it...
homey posted 10-08-2002 01:41 AM ET (US)     Profile for homey  Send Email to homey     
One more thing, the boat is pretty heavy lots of gear and probably abit of water intrusion...Something to think about if your going for more horsepower. If your looking for speed go with the 250hp. My friend has owned this rig since 1995 and runs it pretty hard on the Chesapeake bay. No structural problems/failures with the hull...I believe the limit on his outrage is 150hp.

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