1975 Gulfstar 36’

39dodge · Jun 9, 2024

Good afternoon,
I’m looking at a 1975 Gulfstar 36’ trawler and like it but have some reservations at it has Perkins 4-154’s and the owner says it will only go 5 knots but sips fuel to a time of 3/4 a gal and hr.
Is the above information sound correct?
The max soles of 5 knots scares me if I have to try and avoid the weather in the Gulf of Mexico Mississippi Sound.
Thanks for any inpit on this information and these engines in general.

High Wire · Jun 9, 2024

That’s twin 4-154’s right? I’m sure it will hit hull speed if you need it. Fwiw you’re not going to out run weather at 7 knots either. Plan better.

39dodge · Jun 9, 2024

I understand I’m not to out run the weather but it would be nice to think I have a little more then 5 knots. And yes it is twin 4-154’s

Comodave · Jun 9, 2024

Hull speed should be 1.34 X the square root of the waterline length. You don’t get speed and economy generally in the same boat. If you want a faster boat then you need to look somewhere else.

39dodge · Jun 9, 2024

Compare,
Not looking for a faster boat just can’t believe this trawler will only go 5 knots does that sound right?
I do figure out the square root of the waterline length?

mvweebles · Jun 9, 2024

5-kts cannot be right. My 36-foot boat with a Perkins 4.236 (75 hp) will do close to 7.5 kts but that's pushing it. I normally cruise at 1650 RPM and around 6.5 kts.

A friend's Roughwater 37 with single Perkins 6.354 (135 hp) cruises easily at 7.5 kts though I don't know what WOT speed is.

I don't know the 4.154 well, but would guess it's close to 60hp. Twins should do 7.5 kts without too much trouble.

Peter

39dodge · Jun 9, 2024

mvweebles said:
5-kts cannot be right. My 36-foot boat with a Perkins 4.236 (75 hp) will do close to 7.5 kts but that's pushing it. I normally cruise at 1650 RPM and around 6.5 kts.

A friend's Roughwater 37 with single Perkins 6.354 (135 hp) cruises easily at 7.5 kts though I don't know what WOT speed is.

I don't know the 4.154 well, but would guess it's close to 60hp. Twins should do 7.5 kts without too much trouble.

Peter

That is what I was thinking it might be the sweet spot for the trawler but I was just thinking it has to be a littler bit more the. 5kts

Comodave · Jun 9, 2024

39dodge said:
Compare,
Not looking for a faster boat just can’t believe this trawler will only go 5 knots does that sound right?
I do figure out the square root of the waterline length?

The waterline length will be shorter than the LOA, length overall. Guessing it may be 32 the hull speed should be around 6.5 knots. Hull speed is the theoretical fastest speed. But that speed is not the cruising speed which will be less. Again if you want more speed look at a different boat. Physics is physics…

mvweebles · Jun 9, 2024

Comodave said:
The waterline length will be shorter than the LOA, length overall. Guessing it may be 32 the hull speed should be around 6.5 knots. Hull speed is the theoretical fastest speed. But that speed is not the cruising speed which will be less. Again if you want more speed look at a different boat. Physics is physics…

Agreed, but I think the OP is asking a different question: is 5-kts the right number for a 36-foot boat? FYI - my LWL is 33-feet. 1.0 SL is 5.75 kts. The seller stated it's not even getting to 1.0 SL.

To the OP: on sea trial, the seller should demonstrate the boat running at WOT for 5+ minutes. Some sellers balk which is a walk-away condition for me. It's their boat so their prerogative whether they feel WOT is a problem. A healthy diesel should have no issues running af WOT, though some black smoke isn't unheard of.

Good luck.

Peter

Dougcole · Jun 9, 2024

Our previous boat was a 1976 Gulfstar 36 with twin 4-236, (very reliable motors btw, though they leak oil) and it would happily cruise north of 7 knots. The 4-236 is a bigger engine, obviously, but I didn't have to push them hard to get that speed. We sold the boat in 2012, but if I recall correctly, I ran it around 1,800 rpms.

I would be surprised if you don't get more than five knots.

On the other hand, 3/4 of a gallon per hour seems way too low. I burned 3 gph.

39dodge · Jun 9, 2024

mvweebles said:
Agreed, but I think the OP is asking a different question: is 5-kts the right number for a 36-foot boat? FYI - my LWL is 33-feet. 1.0 SL is 5.75 kts. The seller stated it's not even getting to 1.0 SL.

To the OP: on sea trial, the seller should demonstrate the boat running at WOT for 5+ minutes. Some sellers balk which is a walk-away condition for me. It's their boat so their prerogative whether they feel WOT is a problem. A healthy diesel should have no issues running af WOT, though some black smoke isn't unheard of.

Good luck.

Peter

Thank you for understanding my question. I will do the wide open test on the sea trial.
Again thank you for the assistance

mvweebles · Jun 10, 2024

39dodge said:
Thank you for understanding my question. I will do the wide open test on the sea trial.
Again thank you for the assistance

As a clarification, in my opinion, the seller or their representative should operate the boat during the sea trial, not the buyer. You can ask that they do a WOT test but they may not be comfortable with it.

Good luck

Peter

39dodge · Jun 10, 2024

mvweebles said:
As a clarification, in my opinion, the seller or their representative should operate the boat during the sea trial, not the buyer. You can ask that they do a WOT test but they may not be comfortable with it.

Good luck

Peter

Understand I wasn’t saying I was personally going to do the test I was going ask to have it done. If the owner doesn’t want to do it then I’ll go from there.

kpinnn · Jun 10, 2024

I own that vessel and it very seldom is under 6 knots. There is no problem hitting hull speed at 2250 RPM. I figure 2.5 gph at 2250. Seems to be pretty accurate.

39dodge · Jun 10, 2024

You have 1975 Gulfstar with twin 4-154
Perkins? Can you tell me what should look for as far as common issues? Other than the speed. At 2250 what is the kts you are hitting?

gulfstar3659 · Sep 7, 2024

39dodge said:
Good afternoon,
I’m looking at a 1975 Gulfstar 36’ trawler and like it but have some reservations at it has Perkins 4-154’s and the owner says it will only go 5 knots but sips fuel to a time of 3/4 a gal and hr.
Is the above information sound correct?
The max soles of 5 knots scares me if I have to try and avoid the weather in the Gulf of Mexico Mississippi Sound.
Thanks for any inpit on this information and these engines in general.

Hello i own a gulfstar 36 mark 1 trawler 1974 with 2 perkins 4236 85 hp engines it runs 16,5 km / our , fully loaded fuel and water tank , fully loaded for vacation 2300 rpm with a clean hull. I do not know how much fuel it takes at this speed but it could easely be 15 or 20 ltrs / our.
I never run at this rpm for more than 30 minutes but when i do the engine temp rises no more than 10 dgr celcius ( from 75 to 85 dgr celcius)
Normaly i drive at 1500 rpm = 12 km/our = 6 ltr/ our for both engines together

I did a testrun on sweet water last week
It has a fuel capacity of 953 ltr i send a fotograph with an exact list wich has been made by an official german engineer.
I have a stick wich bends in the middle than you can slide it in the top of your tank and after pulling it out you can measure the wet lenght and check it with the list
I did a complete refit 4 years ago the boat did had some issues but no blisters at all.

Succes

Cargile · Sep 8, 2024

39dodge said:
Compare,
Not looking for a faster boat just can’t believe this trawler will only go 5 knots does that sound right?
I do figure out the square root of the waterline length?

Is that calculation true for all hull shapes?

denverd0n · Sep 8, 2024

If it only goes 5 knots, it is possible that the prop is not at all the correct dimensions for this boat. Also, if the current owner has never bothered to clean the bottom, that could slow it down considerably. Beyond that, the theoretical hull speed for that boat should be north of 7 knots.

But remember that that is a "theoretical" number. Hull speed is not a hard and fast limit. The rule of thumb is 1.34 * square root of LWL, but that is a "rule of thumb." It is not a law of physics. Different hull shapes can give different hull speeds, even with the same LWL. The variation won't be huge from one displacement hull to the next, but there will be some variation.

mvweebles · Sep 8, 2024

denverd0n said:
But remember that that is a "theoretical" number. Hull speed is not a hard and fast limit. The rule of thumb is 1.34 * square root of LWL, but that is a "rule of thumb." It is not a law of physics. Different hull shapes can give different hull speeds, even with the same LWL.

While true that different hull shapes do not cleanly follow the oft-cited S/L ratio calculation, boats owned by TF members are the types that approximately follow the rule. And there is actually a law in physics that governs displacement hull-speed based on distance between the bow and stern waves (wavelength) explained HERE.

I once took a picture of a 50-ish foot pilot boat alongside a ship while a pilot boarded (attached is from Google search - not the best, but you get the idea). Ship was around 500-feet and probably doing 12-14 knots (as was the pilot boat, of course). The ship, going 0.6 S/L, barely disturbed the water while the pilot boat (2.0 SL) was throwing an enormous bow wake.

Peter

denverd0n · Sep 9, 2024

Well, of course physics are involved in determination of the hull speed for any particular boat. What I was saying is that the formula using the number 1.34 is not hard and fast. That is a variable that can be different for different boats. That 1.34 number is going to usually be the right one to get your calculation pretty close to the boat's actual hull speed. That number, however, is not precisely accurate for every single displacement-hull boat in existence.

1975 Gulfstar 36’

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