Estimating speed with HP increase.

[newbster]

My current motor is a bit sick and I'm considering replacement. Is there any good estimate of top end based on HP? The current motor is rated at 170hp and the last time I ran it, WOT was 29mph at 3000 RPM. The motor was not obviously not making full power but I'm guessing 140 hp to the prop at 3000RPM. I may be wrong. The boat itself is rated for 260 HP and I have no performance intel there. In short, the motor still starts and runs fine but might as well go as the boat is under powered. The current motor is a merc 470 and I'm debating a 4.3 mpi or 5.7 mpi. More space and less weight favors the 4.3. Extra power and perhaps efficiency favors the 5.7. I'm looking for efficient but responsive cruise of 20-30 mph in the slop however, power when needed is a good thing.

The boat will fish salt in WA and OR and sees frequent runs 30-60 miles offshore for halibut and tuna. It's primarily a fishing boat. Our weather frequently sucks with flat water being very rare hence the relatively low cruise speed.

I'm trying to guess what 220 hp or 300hp will do with the boat light like the test above on the old motor. Once loaded, or in slop, it will drop but I need to work with something.

 

[bear_69cuda]

Re: Estimating speed with HP increase.

What boat is this?

 

[45Auto]

Re: Estimating speed with HP increase.

In the speed range you're looking at (below 60 MPH), a frequently used rule of thumb is that a gain of 10 HP will give you a 1 MPH increase in top speed. So the 220 HP would give you an additional 5 MPH over it's original top speed with the 170 HP, and the 300 HP would get you a little over a 10 MPH gain.

If your boat came with the 3.7L, either the 4.3 or the 5.7 will easily cruise in the mid 20's. Not sure of the front motor mounts on the 3.7L, but you'll probably have to do some stringer mods to accomadate the front engine mounts on the V6 or V8. You'll also need a different prop.

 

[newbster]

Re: Estimating speed with HP increase.

The boat is a 26' Starcraft Islander.
Est weights light ~ 4000, Heavy- tuna run ~ 5500-6000
Listed hull weight is 2400 which does not include power.
After I started, I realized it becomes a more complicated question. I've heard the 10 hp = 1 mph before so correct me if I'm wrong in saying drag increases substantially with speed so 10 hp increase at a low speed of 30 is not the same as 10 hp at a higher speed of 60. My rough guess would be about 40-44 with 220 hp and a wilder guess of low 50's with the 300.

Given a tuna run is about 150 miles round trip getting an efficient cruise of 25-30 mph could be interesting. Is there reason to think a 5.7 will be more efficient at ~ 180hp?

 

[Mischief Managed]

Re: Estimating speed with HP increase.

HP is necessary to overcome drag. Drag is a function of the square of speed. If you know the actual HP and actual speed, calculating speed from increased HP is easy. Unfortunately, you don't know the actual HP your boat makes since it is not running properly.

That said, assuming the prop is correct for when the boat was running right, we can estimate the top speed to be 45 MPH based on a top operating RPM of 5000 and the typical increase in prop slip as speed increases. So, if indeed you can reach 45 MPH with 170 HP, 260 HP would give you a top speed of 55.6 MPH (assuming the same prop efficiency and weight). You'll lose some speed due to added weight of the bigger engine, but you can make it up with a more efficient prop (think SS).

 

[mike343]

Re: Estimating speed with HP increase.

Did the engine ever run at over 3000 rpm? If not, your prop is way too high in pitch. Before buying a new engine make sure there isn't a fixable problem with it or the prop.

 

[bear_69cuda]

Re: Estimating speed with HP increase.

I think a 5.0 MPI or 5.7 MPI would be great combos in that boat. A carbed 5.7 makes 260 hp I believe... Any of these would be plenty of power for the Starcraft. I would think you would see 50-55 mph with any of these combos.

 

[Philster]

Re: Estimating speed with HP increase.

400 HP pushes 4800 lb performance boats with well-tuned props and aggressive planing hulls to 60. Checkmates, Bajas, etc. All day and night, these things hit the water with 400 HP and they flirt with 60.

300 HP is not going to get that boat from 50-55 MPH. I'd say 45, with bursts of almost 50.

I believe there are some very modern, racy, 25' Caravelle Interceptors with 320 horses pushing along in the low 50's, and a recent Stingray tested out the same: low 50's with 320 HP.

 

[NYBo]

Re: Estimating speed with HP increase.

Going to a V8 will increase weight in the stern. Account for that with trim tabs if you don't already have them.

What is the diagnosis on your current motor? It would be a shame to drop a ton of money on a re-power only to find that something simple is preventing your old motor from running well.

 

[Bob_VT]

Re: Estimating speed with HP increase.

You do realize that if you change the motor you will have to change everything..... including the outdrive gearing.

Possibly Bond-O will chime in. He has refitted a V-6 in his Islander and has been through it.

 

[QC]

Re: Estimating speed with HP increase.

Given a tuna run is about 150 miles round trip getting an efficient cruise of 25-30 mph could be interesting. Is there reason to think a 5.7 will be more efficient at ~ 180hp?

Generally speaking the answer is no. There are exceptions, and your target challenge of 180 hp may be one. However, typically, if all else is equal, the smaller displacement engine and the lighter weight will do better on fuel. Problem is that things are never equal.

Most test data will confirm this. There are a few of us here that recommend scouring Boatest.com for similar weight boats and tests with the same hull and different engines. Do not get suckered into believing the one or two tests that may confirm your initial assumptions. What you need to do is base your decisions on a very wide data set that gives you more of an average conclusion from a variety of combinations. The variables are huge including often forgotten things like ambient temps, fuel temp, weight of the testers, water conditions, a crappy engine from the factory, a crappy hull etc. And then of course other stupid things like a test boat being propped improperly or flippin' typos

 

[45Auto]

Re: Estimating speed with HP increase.

Pay attention to what QC says!

And Philster, you have obviously not been a member of these forums long enough to know that reality is not allowed on these power and speed threads. Please edit your Post #8 above to reflect that a 4 cylinder, 130 HP, 3.0L should push his boat to at least 70 MPH.

 

[Bondo]

Re: Estimating speed with HP increase.

The current motor is rated at 170hp and the last time I ran it, WOT was 29mph at 3000 RPM. The motor was not obviously not making full power but I'm guessing
The boat is a 26' Starcraft Islander.
Est weights light ~ 4000, Heavy- tuna run ~ 5500-6000
Given a tuna run is about 150 miles round trip

Ayuh,.... as Bob said,.. I've got a 74, 221 Islander,...
I yanked the 140hp/ mod.1, then dropped in a 4.3LX/ Alpha 1...
I'll gladly help you through this,..
But,...
I have more questions than answers at this point,....

1, You do realize you'll need to swap out the Entire driveline, not just the motor,... Right,..??
2, What vintage is it,..??
3, How long have you owned it,..??
4, How many of these off shore trips have you made,..??
5, Have you fixed,+ tried the 470,..?? Why not,..??

My barge gained about 10mph, going from the tired 140hp, to the fresh,+ slightly massaged 4.3LX,...
Merc. rated my motor at 205hp,... I'm pretty certain I found every 1 of them,+ put 'em to work...
It'd do about 30mph with the 140hp, Mod.1,...Maybe 34 on a Great day...
With the 4.3l, Alpha 1(1.65:1),+ the torque shift prop, it'll pull a consistent 42/43mph at 4800, here in the Sweetwater Seas...

In All Honesty,...
I believe you'd be much Better off, selling,+ Up-grading to a fiberglass hull, with a deep V....(I'm slowly building a 25' glass replacement now)
Our hulls are just Sooo Flat, they ride absolutely Terribly in the Rough...
Cruising at 30mph just Won't happen, unless the seas are FLAT...
I fish up to 25/30 miles out on Ontario,...
When I go out that far, it's either start out before/ at daylight to get where you're going, cruising at 30mph,...
Or,...
If you leave a 9 or 10:am,... You can count on making 12/15mph, at Best, to keep the Beating to a minimum....
I usually get Sick of the Beatin',+ slow down,+ Troll my way to where I'm going...
Once the sun comes up,+ things start to warm, there's a westerly blowin' in my face,+ 2'/ 3'ers hittin' the bow...

I look forward to your answers,...

 

[Bob_VT]

Re: Estimating speed with HP increase.

Possibly Bond-O will chime in. He has refitted a V-6 in his Islander and has been through it.

Thank you Bond-o

 

[newbster]

Re: Estimating speed with HP increase.

Thanks for the replies.
Bondo:
1, You do realize you'll need to swap out the Entire driveline, not just the motor,... Right,..??

Agreed, it would involve removing and selling whats there and replacing with new or lightly used.

2, What vintage is it,..??
84, it has a 18 degree deadrise, trim tabs and at least 36 inches of freeboard so I think it may not be as bad as you describe. Then again, I have not pushed it. It has only seen moderate chop in the sound with me and it rode decently. Other light boats like Arimas are popular too. I'm stuck by needing to trailer 2-4 ours each way with a 6.0 so I need lighter weight. Saving a hundred bucks plus on gass each run help too. Not having pushed mine under the same conditions leaves room for speculation. The big question is how weight affects ride and I agree lighter = less inertia. My hope is that loading forward and trimming will help quite a bit. Aluminum boats are very popular around here for durability.
3, How long have you owned it,..??
three years, it has seen little use from me as I can jump on many friends boats at will and a house remodel has been bleeding cash. It has seen lake and puget sound use but not offshore as it's not ready yet. It has seen upgrades such as Garmin 4212, gmr18 radar, 2 networked VHFs and fuel flow monitoring and 2 sonars, 4 downriggers etc.. I sprayed the exterior and cabin with Imron, picked up new gauges and glyde rydes for the front seats and a bunch of other upgrades but the motor remains a concern.

4, How many of these off shore trips have you made,..??

This boat has not yet been offshore but I have done many dozens. Realistically, I could keep the boat inshore and fish plenty offshore on friends boats but it's hard to bring my son or other friends along doing so. I'm trying to expand my options with what I have and the weak engine is one concern. My primary goal is to take my son out fishing more often. I have plenty of fish in the freezer from going out with friends and fishing in Mexico etc.. I'm trying to have a workable and safe boat that can reasonably handle the conditions but do so on a "side cash" budget of under 30k. Most all of my friends have dumped 1-300k in their boats and I'm not going that far right now with my son in an expensive school.
5, Have you fixed,+ tried the 470,..?? Why not,..??

The current 470 starts up and runs fine. The compression isn't great at 120/160/135/135 but it saw little use when I took those measurements. technically, it's probably due for a rebuild but it still runs well. Mentally, I'm trying to line up plan b as the 470 is going to be borderline once I throw 100 gallons of gas, 500 lbs of ice and a few guys on it. It just lack safety power thus I question the value in spending more money on it. It seems the 350 is a better power option. I realize that this is a project boat and will continue modify it for fun. I hope your assessment of ride is less valid than you say, is yours also 18 degrees? I thought the older ones were less.

 

[Bondo]

Re: Estimating speed with HP increase.

I'm trying to have a workable and safe boat that can reasonably handle the conditions but do so on a "side cash" budget of under 30k

Ayuh,... Ok,... I'm not sure of the stated deadrise of my hull,+ No, I'm not exaggerating the pizz poor ride...

As long as you have a clear understanding of your hull,... Let's Refit it...

I do refits of classic hulls as a slideline for a couple of Buds,...
Without a Doubt,.. The Best, most inexpensive way to pull it off is,...
Watch the craigslist,+ whatever other resourses you have to find a boat with the exact driveline you Wish your hull had,...
Buy it, then bone it out... That way you'll have ALL the little pieces that drive refit costs through the roof...
You should be able to buy what you want for 5 grand or less...

For what you've got,+ what you want to do with it,.. The 350 efi should be what you're lookin' for...
It'll give you the Power you need at the best economy possible...

As for Drives,... It really don't much matter between the Alpha 1, or the Gen.II,...
The Gen.II has a better impeller pump,+ different shift linkage,.. Durability is about equal...
Of course, a Bravo would be tits... Closed cooling would also be a Big plus...

If, or more likely When you have more Questions,... Fire away,.. I ain't goin' no where......

Bob,... You're more than Welcome,... Thank You for bringing this thread to my attention....
How's the Shovelin' goin',... Heard you guys got 3' the other night.....

 

[GeneH]

Re: Estimating speed with HP increase.

Several posts up, Mike 343 asks about your RPM. Why is the engine only turning 3000 RPM? What is this engine supposed to turn at wide open throttle? What prop is on it? I would think that engine should turn 4600 at WOT. If it should and is only turning 3000, it's horsepower would be way down. Are Mike 343 and I missing something here?

 

[QC]

Re: Estimating speed with HP increase.

I think what you may be missing is that the OP recognizes that the current engine is sick. I am also curious about previous WOT numbers, but the primary reason for the thread was potential top speed and repower info. The whole thing gets a little muddy by trying to glean something out of the current performance numbers, but I believe he was just trying to shed as much light as possible . . .

 

[newbster]

Re: Estimating speed with HP increase.

Sorry about the delay but QC is on the ball as far as my goals. I have not run the boat since tuning the motor up to include rebuilding the carb, had a stuck power valve and replacing the points ignition so it might just reach it's rated rpm of 4200-4400 but it's still a problem as I doubt the boat would reach more than 35 if it's doing 29 at 3000. The issue I face is no matter what I do with the current power, I'm still borderline once I go from 20 gals of gas and two to 100 gallons, 500 lbs of ice and 20-30 20lb fish on the run home. I'm guessing it would have trouble breaking 25mph at wot under those circumstances which is dangerous in swells.

What kills me about the 4.3 is the low end is a couple hundred pounds heavier yet only slightly more power. I would love to get 250 plus hp out of a 4.3 but the technology seems borderline. The next jump is an extra 5 inches and hundred something pounds to the 5.7 to get in the HP range I want. Having extra power if never much of a problem.

Why the heck Mercruiser doesn't put out a corrosion resistant aluminum block engine with HP along the lines of outboards evades me. They could well have made a superior or at least competitive product to outboards and retained market share but they seem bent on destroying their own market.

 

[QC]

Re: Estimating speed with HP increase.

Why the heck Mercruiser doesn't put out a corrosion resistant aluminum block engine with HP along the lines of outboards evades me. They could well have made a superior or at least competitive product to outboards and retained market share but they seem bent on destroying their own market.

Cost and margin. Bugs the heck out of me too as they make a big 4 stroke now . . . I won't less this last point be a hijack if everyone else stays away from it too. I have waaaaaaay too strong of an opinion to not bark up this tree