How Much Does Hull Condition Affect Speed?

[greg82255]

Hello,

I have a 1982 19' Checkmate, did a bunch of engine modifications over the winter, added 30-40 horsepower. My top speed, however, isn't changing. I also painted the boat on the trailer, taped up the rollers and the bottom of the hull. A good amount of tape is still stuck on the hull, haven't been able to get it all off.

Could this be why I am not going any faster? How much does the condition of the hull really affect speed?

 

[cpubud]

Re: How Much Does Hull Condition Affect Speed?

that much of a hp change will not do much,hull condition will slow boat down but unless really bad not that much.

 

[H20Rat]

Re: How Much Does Hull Condition Affect Speed?

Hello,

I have a 1982 19' Checkmate, did a bunch of engine modifications over the winter, added 30-40 horsepower. My top speed, however, isn't changing. I also painted the boat on the trailer, taped up the rollers and the bottom of the hull. A good amount of tape is still stuck on the hull, haven't been able to get it all off.

Did you expect your mods to do anything? Think of it this way, you have a perfectly running setup, now you swap out the engine and drop in one with TWICE as much horsepower without making any other changes. What is your top speed? It will be exactly what it was before. You might have had 50 mph at 5500 rpm before. Your drive is still going to do exactly 55 mph at 5500 input rpm.

If you added power, you also need to change your drive system. Might be as simple as changing prop pitch/diameter/blades. Could be as complex as changing gear ratios. Point is, without changing your drive, you aren't taking advantage of the power.

last point... are you 100% sure you actually added power? I can't count the number of times I've seen someone fiddle with an engine and THINK they add power, only to be told they have less than before. Boat engines in particular don't leave much on the table for extra power and still maintain reliability. Modified car engines don't have to run at 100% engine load for an hour at a time. (and would self destruct long before that anyway!)

 

[greg82255]

Re: How Much Does Hull Condition Affect Speed?

My top speed is about 52 at 4400 RPM, 14.25 x 21 aluminum prop. I have a type I drive with a 1.5:1 ratio. I expected with the engine mods that the RPMs would go up 2-300 and I would bring it back down to 4400 with a 22 or 23" stainless prop. Right now I have the same RPM and speed, no prop change either.

 

[Philster]

Re: How Much Does Hull Condition Affect Speed?

You can add HP and not affect the top speed. Happens every day.

WHERE is the power? Is the power from 3300 to 4000 RPMS? You MIGHT feel a bit more snap in that RPM range. Maybe the 4000-4500 RPM range got 10-15 HP. Maybe it got nuttin'. Sometimes, it gets LESS.

To affect the top end of an engine (the point at where it's trying to push past 4500 RPM in your case) it needs to have:

>Revamped flow for intake. MORE air coming in.
>Revamped flow for exhaust. Be able to scavenge gases when it's hardest, at peak RPM!
>Ability to deliver the fuel under the most demanding condition: Full throttle and max RPM!
> PLUS Something to deliver it to the surface (prop. Maybe Stainless Steel, higher pitch, cupped, etc)

Making cars faster (acceleration) is about adding power (ANY power), because fast cars accelerate faster. It's about dragging, man. Luckily, we don't take 135 MPH cars and try to make them 139 MPH cars, because we would be severely disappointed!! We take cares and try to make them TENTHS of a second faster over ONE QUARTER MILE!

On a boat, we DO worry about 52 MPH versus 55 MPH. A little horsepower runs into massive amounts of physical law too complicated to explore. To get those 3 MPH, something has to DIRECTLY and SIGNIFICANTLY affect the engine at full throttle going 4500 RPMs. You need intake/exhaust/fuel delivery mods, working in unison. However, if the cam and combustion chamber can't work to intake/exhaust anymore than their design allows, you don't get more top end.

She might have a touch more snap somewhere in the RPM range, but to make that engine jump past 4500 RPM, you need to be able to dial in the mods such that they make the HP right there... at WOT.... at the upper part of the power curve.

Not... easily... done.

(I know what you hoped for. You hoped the engine would be too much for that prop, and she'd climb past the RPM wall she hit before. Then you'd get a few more MPH from the next prop up. It's proper to think that way, but it takes a crap load of power delivered at the upper part of the power curve.)

 

[PGFISHER]

Re: How Much Does Hull Condition Affect Speed?

Philster is right. To go twice as fast takes 4x the HP, so adding a little HP probably won't do much for seat of the pants reaction. To feel more speed, takes lots of power.

 

[DJ]

Re: How Much Does Hull Condition Affect Speed?

Hello,

A good amount of tape is still stuck on the hull, haven't been able to get it all off.

Could this be why I am not going any faster? How much does the condition of the hull really affect speed?

Well, heck yes. Going from 52 MPH to anything higher requires a couple things.

1. Super clean "tuned" hull.

2. About another 100 HP.

 

[shrew]

Re: How Much Does Hull Condition Affect Speed?

You state that you painted the hull. Was it painted before? My understanding is that a painted hull can affect performance (albeit slightly), which is why you rarely see performance boats with painted hulls, and many of them are stored on trailers or lifts as a result. The tape isn't going to help either, I'd recommend removing it.

As an example of what has been stated earlier, I repowered my boat from 300Hp to 375HP. Same prop, drive, gearing, etc. What I got was a boat that comes on plane more quickly. I also notice that it doesn't lug down when driving into a wave. The older engine would feel like a car going up hill when going through larger seas (waves 3-4+ footers.) The new engine doesn't seem to notice whether it's going up hill or down hill.

No change in top speed or speed at certain RPM's. The other thing I noticed is it tends to run a bit better at slightly higher RPM's than the older engine, but the speed is relatively the same.

Note, I went from 300hP to 375HP. this is a 25% increase in theoretical HP.

 

[45Auto]

Re: How Much Does Hull Condition Affect Speed?

Making cars faster (acceleration) is about adding power (ANY power), because fast cars accelerate faster. It's about dragging, man. Luckily, we don't take 135 MPH cars and try to make them 139 MPH cars, because we would be severely disappointed!! We take cares and try to make them TENTHS of a second faster over ONE QUARTER MILE!

It's not about dragging unless you're into boat drags. You do realize that there are literally thousands of people around the world who try to make their cars faster (speed, not acceleration)? Ever heard of the Bonneville Salt Flats, Black Rock Desert, or Lake Eyre in Australia? Not everyone is interested in 1/4 mile times.

Fast cars (top speed) don't necessarily accelerate faster. You won't see any Bonneville cars at the dragstrip, or any dragsters at Bonneville. Can't count how many drag races I've seen where a faster car (higher top speed) lost to a slower but quicker car (more acceleration). As an extreme example, which will win in a 1/4 mile drag race: a car that accelerates to 200 MPH within 3 feet of leaving the starting line and maintains that 200 MPH for the rest of the 1/4 mile, or one that takes 1/4 mile to accelerate to 205 MPH at the finish line? Answer is that the slower car (200 MPH speed) will cross the finish line in about 4 or 5 seconds while the faster car (205 MPH) will take about twice as long.

Making a car go 139 MPH instead of 135 MPH is no different than making a boat go 55 MPH instead of 53 MPH. All it takes in either case is a given amount of additional power to overcome drag. Takes more horsepower in the case of the boat because water drag is MUCH higher than air drag.

A good general rule for 50-60 MPH boats is that it takes 10 HP to gain 1 MPH in speed. So if the original poster really added 20-30 HP to his motor the most difference he would see is 3 MPH. This could be easily overcome by atmospheric conditions (temp & humidity, wind direction, etc), boat weight (extra person, gear or fuel in the boat), or boat weight distribution even if the total weight doesn't change.

 

[Philster]

Re: How Much Does Hull Condition Affect Speed?

It's not about dragging unless you're into boat drags. You do realize that there are literally thousands of people around the world who try to make their cars faster (speed, not acceleration)? Ever heard of the Bonneville Salt Flats, Black Rock Desert, or Lake Eyre in Australia? Not everyone is interested in 1/4 mile times.

Fast cars (top speed) don't necessarily accelerate faster. You won't see any Bonneville cars at the dragstrip, or any dragsters at Bonneville. Can't count how many drag races I've seen where a faster car (higher top speed) lost to a slower but quicker car (more acceleration). As an extreme example, which will win in a 1/4 mile drag race: a car that accelerates to 200 MPH within 3 feet of leaving the starting line and maintains that 200 MPH for the rest of the 1/4 mile, or one that takes 1/4 mile to accelerate to 205 MPH at the finish line? Answer is that the slower car (200 MPH speed) will cross the finish line in about 4 or 5 seconds while the faster car (205 MPH) will take about twice as long.

Making a car go 139 MPH instead of 135 MPH is no different than making a boat go 55 MPH instead of 53 MPH. All it takes in either case is a given amount of additional power to overcome drag. Takes more horsepower in the case of the boat because water drag is MUCH higher than air drag.

A good general rule for 50-60 MPH boats is that it takes 10 HP to gain 1 MPH in speed. So if the original poster really added 20-30 HP to his motor the most difference he would see is 3 MPH. This could be easily overcome by atmospheric conditions (temp & humidity, wind direction, etc), boat weight (extra person, gear or fuel in the boat), or boat weight distribution even if the total weight doesn't change.

I was explaining that cars are about dragging. People add power to cars for improved acceleration, and generally don't take their cars and do repeated top speed runs trying to find another 2-3 MPH. They want more grunt to accelerate. It's about dragging (for cars).

Many take that approach ("Hey, I just added 30 HP to my boat engine") and head out on the water for a top speed run, or two, or three, or four. Doesn't work like that on the street. So, taking the street rodder's approach (aimed at acceleration) to build top speed in a boat doesn't usually translate well.

I also think that it is incredibly optimistic to look for a 1 MPH increase in top speed when adding a scant 10 HP, because we don't know where in the power curve that 10 HP went. 10 HP might mean a 2% total increase in performance overall, rather than a 2% increase in top speed.

Some HP upgrades don't help the torque curve. That a whole nudder story.

 

[Mischief Managed]

Re: How Much Does Hull Condition Affect Speed?

I don't know what your original HP was so the following is simply an example.

If you went 52 MPH with 250 HP, the boat should be able to go 56 MPH with 290 HP, if drag and prop efficiencly have not changed. Ablative bottom paint will slow you down, but an 8% reduction is pretty excessive for just paint, so I'm guessing you did not add much HP.

 

[45Auto]

Re: How Much Does Hull Condition Affect Speed?

I also think that it is incredibly optimistic to look for a 1 MPH increase in top speed when adding a scant 10 HP, because we don't know where in the power curve that 10 HP went.

I've NEVER seen a post on any boat forum where anyone referred to anything but peak horsepower when talking about power increases. It would be interesting to see any other references or links you may have.

In common usage adding 30 HP to a 220 HP engine gives you a 250 HP engine. I've never seen any references to power at an RPM less than WOT RPM. Lots of references to additional TORQUE at less than WOT RPM, which will give you additional potential available power at the lower RPM. The only way to add power at any particular part throttle RPM is to add TORQUE. The added torque will give you more power available, therefore additional acceleration, at that RPM when you nail the throttle.

Adding additional PEAK power no matter what RPM it's at will result in a faster boat if geared/propped correctly. A motor that you modify from 220HP at 5000 RPM to 250 HP at 4600 RPM (but still falls off to 220HP at 5000 RPM) will be faster than the 220HP motor in top speed IF you prop it to turn 4600 RPM at WOT. Leave the same prop on it as the 220 HP motor and they both will go the same top speed at 5000 RPM, although the 250HP motor will get there a hair quicker. Same thing (but opposite direction in prop pitch) will apply if you add power higher up in the RPM range.

Either of these cases could easily be the situation with the original poster's motor if he truly added 30 HP. You'll find that most of the time the "additional horsepower" is pretty much a figment of somebody's imagination, unless they can rationally describe exactly what they did to modify their engine for increased output.

Without a dyno chart of the before and after power output of the motor or one with similar mods, you have no idea which way you need to go. Is the RPM where the new peak power is located below, the same, or above the RPM where the original peak power was located? Most people don't have access to a dyno and wouldn't know how to use the info anyway. Boaters seeking this magic spot keep a LOT of prop companies in business!