Power adders for running underpowered boats at "high altitude", aka Lake Powell

Hey all. There isn't really a "motorhead" section to this forum, and we are primarily recreational boaters, so I figured I'd give it a shot.

We have a Sea Ray Sundeck 240 2005, with the Bravo 3 behind a 5.0 Mercruiser. All my prior experience was with Alpha drives, and for the most part I love the Bravo, especially in reverse in a tight slot, not so much trying to walk around into the wind, amazing performance in shallow water with it trailered. Coming out through the really bad water 3 weeks ago took our gimbal from loose to sloppy, so were off the lake for a month while I rebuild it. (would coulda shoulda done it over the winter)

The 5.0 is a fantastic motor, most of the time. We get pretty excellent mileage, about 11-12gph at 27-30mph cruising, 7-8gph water sports. Top speed of maybe 37 at 15.5gph with an empty boat, the Bravos don't seem to shine at higher speeds.

The issue we run into is getting on plane with a full load. Once planing we are ok, unless we get into really bad chop (like the channel May-September), then it gets a little dodgy, the big bow likes to "scoop" if you come off plane(looking for a bow cover, gimbal repair first). In the last bad wind storm, we were at maybe 60% load, and the margin of performance was reassuring. We are running Hill Marines 4x4 20 pitch props, a huge improvement from the stock 21 3 blades, really improved the performance and handling of the boat, and improved our margin. Big vote of confidence for Hill. Great service and very fast shipping. I'm not excited about further reducing to the 18 pitch, as I really don't want to give up my cruise performance, and RPM wise we are about right with these props. (I underpitched the last Alpha, great for launching, but you had to manually limit rpm.)

So, Power Adders Has anyone experimented with light nitrous dry or wet shots for short term power, 20 sec or so to get the motor higher in the horsepower curve? Maybe 50hp? Bottle and piping wouldn't add much weight and we have space. Engine isn't really prepped for nitrous, and speed isn't the goal, my hope is a light enough shot doesn't require timing changes, and just makes up for the lack of atmospheric pressure, and the fuel injection can adapt and richen. I also have other projects that I'd like to try nitrous on, so the costs can be spread a little. (cinders hill climb truck, clearly a bad idea, isn't that the point?)
My other thought was a manifold for 3-4 12v blower fans into the intake. Would not be a turbo, but might compensate for the altitude somewhat? A belt driven supercharger is a real solution to the same idea, but I don't want to run it all the time, and I'm not sure how they would do with an electric clutch on them (and fairly expensive). The Mercruiser 6.0 would be perfect, but unless I can find a wreck, would cost a high percentage of the boats worth.

I realize that the rational answer is to take less stuff fewer people or stay fewer days, but with Dangling Rope closed, the fuel jugs alone put us heavy, and we want our cake and to eat it too.
 

Dobbs

Member
Propping is your best bet. Trying to super charge or put nitrous on a 5.0L is a really bad idea. Your better doing a repower if you want more performance or buying a new boat. Check the rating on the boat and don’t try to push more than what it was designed to do. I would take it to a certified mercury mechanic and talk to them
 
Propping is your best bet. Trying to super charge or put nitrous on a 5.0L is a really bad idea. Your better doing a repower if you want more performance or buying a new boat. Check the rating on the boat and don’t try to push more than what it was designed to do. I would take it to a certified mercury mechanic and talk to them
Fair enough. The boat did come speced with the 350 mag option as well, so power wise I think the repower would be ok. At Powell the 6.0 would make less than the 350 at sea level. We are really shy on boat mechanics in Northern AZ. The one worth talking to is usually booked to September by the end of May, one reason I'm doing my own Gimbal, since I don't actually enjoy banging my head on the swim platform.
 

Dorado

Escalante-Class Member
Hahaha Lake Powell a " high elevation lake"! My local lakes are 7,400-8,000 feet, there is no getting around the need to re-prop when changing elevation. Sounds like you already know this, but you are going to have to sacrifice some top end speed. But you should be able to plane out and haul a bunch of stuff with a prop with less pitch...
 
I live at 7000, and my lungs work great! at Powell.

I see a bigger boat in my future, but need to arrange a bigger paycheck first. Love the big Cobalts. Nice lines.

I need to talk to Hill about options. They may be able to take a little of the bite out of my current set, so not as big a compromise.
 

Turbo

Member
If you’ve got a bravo drive, replace the 5.0 with a big block.
That little 5.0 DOES NOT have the bottom end to take boost, and doesn’t have enough ring end gap for nitrous. By the time you open it up and make it capable, you can dump a big block in it and have what you want, and have it reliable.
 

Leardriver

Well-Known Member
I have had 4 240's and one 270. Absolutely terrific Lake Powell rough water boats.
At sea level, you can survive the 5.0 if all you do is pull a tube. That boat should never have been offered with a 5.0. It ruins what is otherwise a great boat, maybe the greatest family open bow ever made.
You can gear around it, and kill top speed. That works fine.
Power adders like nitrous run out quickly. I am not a fan on a boat. A bomb planted under a hot compartment next to fuel, operating in an environment where you drown is stacking up undesirable choices.
I ran 22 pitch props on the 240's with a 6.2 or a 350. Both of those motors are adequate. High altitude and heat just kill the air density and you lose 20%+ horsepower.
When I had my 270 with a 496 and an 85 gallon gas tank, that was close to a perfect Powell boat.

The Cobalts have great build quality, but lack some of the creature comfort of the Sea Ray, like the sink and cooler area in the back.

270 interior layout.
 

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Terry

Member
I have a 28 foot cruiser with small block 300 hp and have found that running pure fuel (no ethanol) and adding Startron adds a little extra pep and gets me on plane a little faster. Evey little bit helps when your under powered and over weight. (Talking about the boat ofcourse)
 

Kevin G

Well-Known Member
My boat mechanic’s theory was always buy a boat that is overpowered, then to maintain your speed you don’t have to work the engines nearly as hard. People always worry about fuel economy with big block engines but if don’t have to wrap up the engine to maintain a decent cruising speed, fuel economy really isn’t that bad. In the end, more than hours, number of revolutions is the best measure of engine wear and tear. If you’re running your engine at 4,500 rpms to cruise at 30-35 mph, you’re better off with a big block or a pair of big blocks.

Sea Ray 320 Sundancer with twin 496s getting almost 40 mph at 4,000 rpm’s.


AC4AB835-F322-4050-8BDB-CBD30805760F.jpeg
 
I would just pick up a 5.7 or 6.2 marine long block from blue print engines. Everything will be plug and play with the 5.0 all you have to do is get the ecm reflashed. A lot more bang for the buck and we help your resale rather than hurt it with the nitrous.
Ok, this is interesting! I need to look into that. I had heard that the exhaust manifolds weren’t compatible. Now that I think on it though, all the small blocks share architecture, and they should bolt right up.
 

Havalina

Escalante-Class Member
Look at procharger. I live at 6000 feet, and ran into issues with my gutless 4runner barely being able to pull a 3500 lb trailer up a 2000 ft mountain pass. My hp jumped to just under 500 hp and I doubled my gas mileage. I know that procharger makes superchagers for marine purposes. I know apples and oranges, but worth a look. I know propping also makes a big difference. Both pf my boats are propped at 13 with my average elevation at 7500 ft.
 

sphoppie

Active Member
Ok, this is interesting! I need to look into that. I had heard that the exhaust manifolds weren’t compatible. Now that I think on it though, all the small blocks share architecture, and they should bolt right up.

Here who I used for my engine.

BluePrint Engines 383CI Stroker Marine Crate Engine | Small Block GM Style | Longblock | Iron Heads | Roller Cam

Just need to make sure you get the required
harmonic balancer and fly wheel.

This guy can reflash your ecm.
 

Powelldreamer

Well-Known Member
I will echo what has been recommended as far as repower. The 5.0 is just not cut out to properly run a 24 footer. I have a 5.7 in my Sunesta 252 deck boat, I really need the big block. The small blocks just have to work too hard for this size boat. I wish I had a 7.4.
 

Lake Bum

Well-Known Member
You will completely trash that 5.0 with trying to pull more horses out of it. I would suggest adding some Lenco trim tabs to your boat, to help with that low end, outta the hole help. Do NOT get the "smart tab" options on other brands, they are worthless, I speak from experience. Lenco's have electric trim tabs, Bennet's come in hydralic, both are great, and a lot cheaper that a re-power. If you do re-power, you'll need to verify your drive has the proper gear ratio to take a higher power big or small block. I have a Rinker 232 cuddy, that has the 350 MAG/Bravo 3 combination, and that is a perfect power/weight ratio for Powell. I run 3 blade 24's and have tons of low-end holeshot power.
 
You will completely trash that 5.0 with trying to pull more horses out of it. I would suggest adding some Lenco trim tabs to your boat, to help with that low end, outta the hole help. Do NOT get the "smart tab" options on other brands, they are worthless, I speak from experience. Lenco's have electric trim tabs, Bennet's come in hydralic, both are great, and a lot cheaper that a re-power. If you do re-power, you'll need to verify your drive has the proper gear ratio to take a higher power big or small block. I have a Rinker 232 cuddy, that has the 350 MAG/Bravo 3 combination, and that is a perfect power/weight ratio for Powell. I run 3 blade 24's and have tons of low-end holeshot power.
I do have the factory Bennet hydraulic tabs on the boat. They help a little, but strike me as kinda small, and fully deployed the drag is significant. I put the same size (~12'x12") on my sisters 18' and they were damn effective. I've been considering digging some aluminum out of the cowboy bank account and making progressively larger bolt on tabs to see if it helps and gives me lift without being at as extreme an angle.
 
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