I want better speed and performance

[Skidude17]

I have 1995 Sea Ray Overnighter that weighs in at 5,000 lbs with people full tank and gear.
I currently have the stock aluminum mercury 48-78118 17P (14.5” 17P) prop behind the 5.7L MCM Alpha1 Gen 2 235 hp
I’m maxing out at 38 mph (gps) and 4800 rpm (boat tach) at WOT. I had one instance where I trimmed up a little more without porpoising, but I gained rpm and lost speed.
Gear ratio 1.61

I’m hoping to switch to a stainless prop to get better top end speed and gas mileage for long cruises to the San Juans.

My mechanic recommended a Mirage 3 blade 15.75” 15P.

Does this seem like the right way to go?

 

[achris]

My experience with Mirage is that they have very high bow lift, and will hold the bow up as long as possible. On my boat, that produced a slamming effect, I hated it..

Fuel usage isn't going to change much between props, a little higher with 4 or 5 blade, that's about all.

My prop calculator (I wrote the program myself) suggests you have the right prop on now, and so does the Merc prop selector. It is calling out to go a size up (19") for midrange/fuel economy, but staying with what you have for top speed. Nowhere does it recommend a Mirage, and neither would I. Are you sure your mechanic isn't pushing the Mirage because he has one on the shelf he wants to sell?

Chris.........

 

[Skidude17]

Are you sure your mechanic isn't pushing the Mirage because he has one on the shelf he wants to sell?

Chris.........

He very well may be. He is a Merc dealer... So you don’t think switching to stainless will give me any better performance? I love making long runs with her, but the gas bill is huge, and going that far, I’ll have to fill up at a marina!

I’m guessing it’s 19” pitch? Would that be keeping the same diameter?

 

[achris]

The 19" will have a slightly smaller diameter (about 1/4" less). Switching to stainless will improve it slightly, 1, maybe 2 knots, at the most. As for fuel, it uses what it uses. Not much can be don't to change that. I did a series of test runs with my boat using 4 different props, and for the same boat speed, the fuel use was the same.

Chris.....

 

[Skidude17]

The 19" will have a slightly smaller diameter (about 1/4" less). Switching to stainless will improve it slightly, 1, maybe 2 knots, at the most. As for fuel, it uses what it uses. Not much can be don't to change that. I did a series of test runs with my boat using 4 different props, and for the same boat speed, the fuel use was the same.

Chris.....

Thanks.

 

[Scott Danforth]

generally when you are maxed out currently with power, to get more speed and performance, you need more motor

your current prop gets you to your max WOT RPM. as Chris mentioned, you may be able to eek out another 1 or two mph and drop your WOT a slight bit.

fuel economy is directly related to the amount of power you are applying. that is a constant. its called BSFC (Brake Specific Fuel Consumption). to get better economy, you need to back off on the power (speed) or go diesels

your motor burns 0.4 # of fuel per HP per hr
a diesel burns 0.333 # of fuel per HP per hr

to get better fuel economy in a boat usually involves sails or slaves and oars.

 

[Sea Rider]

What's the max wot rpm range for that motor and max passengers rated for that boat, with how many *with people* might that be ? Will be running most of the time a fixed or variable passenger load along same load with said combo.

Happy Boating

 

[Scott Danforth]

What's the max wot rpm range for that motor and max passengers rated for that boat, with how many *with people* might that be ? Will be running most of the time a fixed or variable passenger load along same load with said combo.

Happy Boating

Sea Rider, he is already at max WOT on his motor. he can go up just a bit in prop, however the OP is out of motor to get any more speed. big boats need bigger power

 

[jimmbo]

Sea Rider, he is already at max WOT on his motor. he can go up just a bit in prop, however the OP is out of motor to get any more speed. big boats need bigger power

I agree fully on that. Given a choice of a 325hp Small Block or a 325hp Big Block I would pick the Larger Displacement every time if the Boat was longer than 23 ft, at least it would get on plane in less than 2 minutes

 

[Sea Rider]

Fully agree on that one too, if wanting to increase the top end speed he should dump at terra firme some of his fellow boaters as the OP has not specified with how many boaters he likes going boating with which will be good to know. Excessive weight while the boat is powered by an out of motor one is a top speed serial killer..

Happy Boating

 

[gm280]

And like so many other things, to gain more speed, you have to really add more HP. It isn't a simple linear scale. And a mere ten more HP would probably be totally unnoticeable for sure. It takes a whole lot more HP to see any noticeable difference in speed...

 

[jimmbo]

The old Rule of Thumb was 1 mph for every 5 hp, with the exception being where the added power was enough to get a Planing Hull, planing

I will assume that RoT may also go out the Window on Boats bigger than 23 ft or so

 

[jimmbo]

Fully agree on that one too, if wanting to increase the top end speed he should dump at terra firme some of his fellow boaters as the OP has not specified with how many boaters he likes going boating with which will be good to know. Excessive weight while the boat is powered by an out of motor one is a top speed serial killer..

Happy Boating

Putting the Wife, Kids, and Mother in Law(especially the Mother in Law) ashore was always a good Idea, and adds to the Boating experience. Girlfriends and Mistresses can remain aboard, for the same reasons

 

[JimS123]

I have yet to own a boat that didn't see an improvement in speed and economy when switching to a SS prop.

Was it a night and day comparison?.....No. Was it noticeable to the naked eye?...No. Did the data bear it out?...Yes.

The key is to have the WOT rpm right at the max spec for the motor.

We all have different boating lifestyles. Sometimes we take a lady with us, sometimes we take the whole family. Obviously, no single prop will satisfy us all the time. Pick one for the average conditions and make do with all the other ones.

One of my buddies insists on having the fastest boat on the river and he spends countless dollars ensuring that. We just plod along and end up having more fun than he does because we are not bothered.

 

[Skidude17]

What's the max wot rpm range for that motor and max passengers rated for that boat, with how many *with people* might that be ? Will be running most of the time a fixed or variable passenger load along same load with said combo.

Happy Boating

My wife and 2 kids with all their requirements are pretty much a fixture.

 

[QBhoy]

I’d stick to a 17” good stainless like a laser 2, tempest or similar. Wouldn’t go to a 15” or a 19” if I was you.

All the best.

 

[Scott Danforth]

My wife and 2 kids with all their requirements are pretty much a fixture.

I wouldnt ask you to keep the Admiral and swabbies ashore



The issue is your 250 DA has a 5000# dry weight. add 70 gallons of fuel, 20 gallons of water, fill the holding tank, have cooking gear, bedding gear, etc. and your pushing closer to 6500#-7000#.

base engine was the 235hp 5.7 and alpha that you have. this is similar power to weight ratio of a bayliner 2250 with a 3.0. it gets you on the water, it just doesnt have enough motor

IIRC, your boat probably takes about 30+ seconds to get out of the hole.

optional engines ranged from carbed to EFI 5.7s and 7.4's and optional drives were the Bravo 1,2, and 3's

your boat really needs a 300hp motor (or more) and a Bravo 3. the extra power to spin the extra bite needed.

However at a minimum, I would toss on a set of vortec heads and maybe a set of 1.6:1 or 1.7:1 rockers and get an additional 30+hp out of your current motor, and optimize a prop after that.

 

[Stinnett21]

How far is it to the San Juans? You mean you run that old set up WOT for that long? Got more guts than me. Looks like you've even have a kicker on the back. Previous posts are correct. To satisfy your desires of more speed and better fuel economy you either need larger engine or a different boat. Here's what I would do if I had your set up and I couldn't afford the aforementioned fixes. I would time my trips to where speed isn't the concern. Heck isn't getting there most of the fun anyway? I can't tell from your pic but I haven't heard trim tabs mentioned. No they won't gain you speed but will certainly gain better economy as you could throttle back a bit and enjoy the ride.

 

[QBhoy]

Is your engine not the 250/260hp at that age ?
anyway....some guys here have went for the big 16x16 black max ally prop, with some success. Similar top speed to the 14x17” black max but way way better out the hole.
I know of a bayliner 2855 with a 5.7 250 and alpha...he has one and I was seriously impressed with it out the hole for such a big boat.

 

[Sea Rider]

My wife and 2 kids with all their requirements are pretty much a fixture.

Good to know, prop the motor right with whichever prop model you like going for to run towards its max wot rpm range factory stated. At full hammer down, the combo should achieve a fantastic hole shot, once on plane reduce throttle enough to maintain a perfect plane along reduced fuel consumption.

Most boaters have the wrong idea that a boat lightly or fully loaded will perform opt as a car riding with same load on a flat highway. The tire asphalt drag compared to a boat's hull riding on water is bare minimum, the other one is huge.

Happy Boating